Give read-aloud a Portuguese voice, and a slower one

Phase 21's infra half. Two things the pt-PT pair needs from TTS, and one
thing every learner has wanted since Phase 11.

**A language is no longer a code change.** The handler knew exactly two
languages, named in the Config struct: English on TTS_ENDPOINT and Chinese
on TTS_ENDPOINT_ZH. Petal now discovers its Piper instances from the
environment — English keeps the unsuffixed pair it has always had, and
every other language is a TTS_ENDPOINT_<LANG>/TTS_VOICE_<LANG> pair — so
fr and es cost a compose service and two lines of .env. <LANG> is the base
tag, because an environment variable name cannot hold pt-PT's hyphen and
only one Portuguese model is loaded either way. A language configured by
halves is dropped rather than routed: half a configuration should reach
the client as "no voice here, use Web Speech", not as an instance that
errors on every tap. The startup line now names the voices it actually
resolved rather than the English endpoint it was handed — the same lesson
the dictionary line learned last week.

**pt_PT-tugão-medium is the only European voice Piper ships.** The other
five pt models in the catalogue are Brazilian, so the default anyone
reaches for is the wrong country — the same trap as `dictionary-pt`
packaging VERO, arriving through the catalogue rather than through the
model. Named explicitly in compose, with the query that checks it in the
deploy README.

**The slow replay** (SUGGESTIONS §5e) is `slow: true` on /api/tts, raising
Piper's length_scale to ~4/3. Piper stretches durations rather than
resampling, so it stays a voice instead of a groan. The pace is part of
the cache key — without it the slow replay of a word already heard at
normal speed would be served back at normal speed, which is the one
request where the difference is the whole point. 🐢 sits beside 🔊 on the
word card, the selection bubble and the garden flashcard; the Web Speech
fallback slows too, so the button means the same thing when Piper is down.

**And the other reading gets her own voice.** The `alsoIn` block — the
Portuguese sense of a word that is also English — now speaks in the pair's
locale, which the pack names (`locale`) rather than anything inferring it
from the letters. "comum" is spelled identically in both halves; a
detector would have to guess, and this is the same reason the gloss shows
both directions instead of picking one.

Tests: config discovery (both existing deployment shapes, half-configured
languages dropped, the pre-map voice defaults preserved), the slow scale
and its separate cache entry, pt routing on the base tag with pt-BR
landing on the European instance, and speech.ts's request body. The i18n
shape suite now asserts every pack names a speakable locale in its own
language — and that pt-PT's is not pt-BR.

Verified: go build/vet/test, tsc, vitest 125/125, vite build. Live smoke
against two fake Piper servers: en/pt × normal/slow all reached the right
instance at the right length_scale with four distinct cache entries, and
an unconfigured language still 404s.

internal/tts/handler.go

@@ -19,6 +19,7 @@ import (
 	"os"
 	"os/exec"
 	"path/filepath"
+	"sort"
 	"strings"
 	"time"
 	"unicode/utf8"
@@ -36,6 +37,12 @@ const maxTextBytes = 4000
 // with the words, mirroring the old utterance.rate = 0.95. Higher = slower.
 const lengthScale = 1.1
 
+// slowLengthScale is the "say it slower" replay (SUGGESTIONS §5e): roughly 0.75×
+// the normal pace, which is the speed listening drills have used for decades.
+// Piper stretches durations rather than resampling, so the voice keeps its pitch
+// instead of turning into a slowed tape.
+const slowLengthScale = lengthScale / 0.75
+
 // audioFormat describes one output encoding: the cache-file extension, the
 // response Content-Type, and the ffmpeg args that turn Piper's WAV (on stdin)
 // into this format (on stdout). A nil ffmpegArgs means "serve the WAV as-is".
@@ -105,16 +112,14 @@ func New(cfg *config.Config) (*Handler, bool) {
 		format = formats["mp3"]
 	}
 
-	// Map by base language so en-US, en-GB, etc. all resolve to the English
-	// instance (the client sends BCP-47 tags like the old Web Speech path did).
-	// A language is only routable when both its endpoint and voice are set;
-	// otherwise the client falls back to Web Speech for that language.
+	// Keyed by base language so en-US, en-GB — and pt-PT, pt-BR, bare pt —
+	// resolve to the one instance that has that language's model loaded (the
+	// client sends BCP-47 tags, as the old Web Speech path did). Config has
+	// already dropped any language configured by halves, so an unroutable
+	// language reaches the client as a 404 and falls back to Web Speech.
 	routes := map[string]route{}
-	if cfg.TTSVoiceEN != "" {
-		routes["en"] = route{strings.TrimRight(cfg.TTSEndpoint, "/"), cfg.TTSVoiceEN}
-	}
-	if cfg.TTSEndpointZH != "" && cfg.TTSVoiceZH != "" {
-		routes["zh"] = route{strings.TrimRight(cfg.TTSEndpointZH, "/"), cfg.TTSVoiceZH}
+	for lang, v := range cfg.TTSVoices {
+		routes[lang] = route{endpoint: v.Endpoint, voice: v.Voice}
 	}
 
 	if err := os.MkdirAll(cfg.TTSCacheDir, 0o755); err != nil {
@@ -132,6 +137,19 @@ func New(cfg *config.Config) (*Handler, bool) {
 	}, true
 }
 
+// Languages lists the base language tags this handler can synthesize, sorted,
+// each with the voice serving it — for the startup line, so a deployment says
+// which sidecars it actually reached rather than which ones it was configured
+// to want.
+func (h *Handler) Languages() []string {
+	out := make([]string, 0, len(h.routes))
+	for lang, rt := range h.routes {
+		out = append(out, lang+"="+rt.voice)
+	}
+	sort.Strings(out)
+	return out
+}
+
 // Routes mounts the synthesis endpoint. Mount under "/tts" so the full path is
 // POST /api/tts.
 func (h *Handler) Routes() chi.Router {
@@ -140,11 +158,13 @@ func (h *Handler) Routes() chi.Router {
 	return r
 }
 
-// synthRequest is the body the editor posts: a passage and the BCP-47 language
-// tag it's written in (e.g. "en-US", "zh-CN").
+// synthRequest is the body the editor posts: a passage, the BCP-47 language tag
+// it's written in (e.g. "en-US", "zh-CN", "pt-PT"), and whether to say it slowly
+// — the replay a learner reaches for when the sentence went past too fast.
 type synthRequest struct {
 	Text string `json:"text"`
 	Lang string `json:"lang"`
+	Slow bool   `json:"slow"`
 }
 
 // synth resolves a voice for the requested language, returns cached audio when
@@ -180,9 +200,17 @@ func (h *Handler) synth(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	// Content-addressed: identical (voice, text) → identical clip. The format
-	// extension keeps encodings from colliding in the same dir.
-	sum := sha256.Sum256([]byte(rt.voice + "\n" + text))
+	scale := lengthScale
+	if req.Slow {
+		scale = slowLengthScale
+	}
+
+	// Content-addressed: identical (voice, pace, text) → identical clip. The pace
+	// belongs in the key — without it the slow replay of a word already heard at
+	// normal speed would be served from cache at normal speed, which is the one
+	// request where the difference is the whole point. The format extension keeps
+	// encodings from colliding in the same dir.
+	sum := sha256.Sum256([]byte(fmt.Sprintf("%s\n%.3f\n%s", rt.voice, scale, text)))
 	name := hex.EncodeToString(sum[:])[:32] + h.format.ext
 	path := filepath.Join(h.cacheDir, name)
 
@@ -191,7 +219,7 @@ func (h *Handler) synth(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	audio, err := h.synthesize(r.Context(), rt, text)
+	audio, err := h.synthesize(r.Context(), rt, text, scale)
 	if err != nil {
 		http.Error(w, "synthesis failed", http.StatusBadGateway)
 		fmt.Fprintf(os.Stderr, "tts: synthesize: %v\n", err)
@@ -222,11 +250,11 @@ func (h *Handler) serve(w http.ResponseWriter, r *http.Request, path string) {
 
 // synthesize POSTs to the route's Piper instance, then transcodes the returned
 // WAV when the configured format calls for it.
-func (h *Handler) synthesize(ctx context.Context, rt route, text string) ([]byte, error) {
+func (h *Handler) synthesize(ctx context.Context, rt route, text string, scale float64) ([]byte, error) {
 	body, _ := json.Marshal(map[string]any{
 		"text":         text,
 		"voice":        rt.voice,
-		"length_scale": lengthScale,
+		"length_scale": scale,
 	})
 	httpReq, err := http.NewRequestWithContext(ctx, http.MethodPost, rt.endpoint+h.synthURI, bytes.NewReader(body))
 	if err != nil {