Fix correctness bugs found by code review

- matrix: drop cryptohelper LoginAs so device.json creds aren't
  invalidated on every restart by Init's re-login
- matrix: thread ctx through PostThreadedReply; bot dispatcher derives
  per-command ctx from app lifecycle so SIGTERM cancels in-flight work
- matrix: isTokenValid verifies /whoami user_id matches configured one
- matrix: loadDevice distinguishes missing file from corrupt parse;
  refuse to silently overwrite a corrupt device.json
- matrix: Start checks Syncer type assertion and returns error
- arr: parseLookup skips empty-title items and extracts existing id
- bot: skip Add when Result.Exists; reply "already in library"

internal/bot/bot.go

@@ -19,9 +19,10 @@ import (
 	"maunium.net/go/mautrix/id"
 )
 
-// Replier sends a threaded reply into roomID under rootEventID.
+// Replier sends a threaded reply into roomID under rootEventID. The ctx
+// governs cancellation/deadline so shutdown propagates through replies.
 type Replier interface {
-	PostThreadedReply(roomID id.RoomID, rootEventID id.EventID, plain, htmlBody string) error
+	PostThreadedReply(ctx context.Context, roomID id.RoomID, rootEventID id.EventID, plain, htmlBody string) error
 }
 
 // Services holds the configured *arr clients. Any field may be nil; commands
@@ -34,6 +35,7 @@ type Services struct {
 
 // Dispatcher routes parsed commands to services and posts replies.
 type Dispatcher struct {
+	baseCtx  context.Context
 	prefix   string
 	services Services
 	replier  Replier
@@ -41,12 +43,17 @@ type Dispatcher struct {
 }
 
 // New builds a Dispatcher. prefix is the leading character(s) on a command
-// (e.g. "!"); a zero value defaults to "!".
-func New(prefix string, services Services, replier Replier) *Dispatcher {
+// (e.g. "!"); a zero value defaults to "!". baseCtx becomes the parent of
+// every per-command context so app shutdown cancels in-flight handlers.
+func New(baseCtx context.Context, prefix string, services Services, replier Replier) *Dispatcher {
 	if prefix == "" {
 		prefix = "!"
 	}
+	if baseCtx == nil {
+		baseCtx = context.Background()
+	}
 	return &Dispatcher{
+		baseCtx:  baseCtx,
 		prefix:   prefix,
 		services: services,
 		replier:  replier,
@@ -64,11 +71,11 @@ func (d *Dispatcher) Handle(roomID id.RoomID, eventID id.EventID, sender id.User
 
 	slog.Info("bot: command received", "room", roomID, "sender", sender, "cmd", cmd, "query", query)
 
-	ctx, cancel := context.WithTimeout(context.Background(), d.timeout)
+	ctx, cancel := context.WithTimeout(d.baseCtx, d.timeout)
 	defer cancel()
 
 	plain, htmlBody := d.dispatch(ctx, cmd, query)
-	if err := d.replier.PostThreadedReply(roomID, eventID, plain, htmlBody); err != nil {
+	if err := d.replier.PostThreadedReply(ctx, roomID, eventID, plain, htmlBody); err != nil {
 		slog.Error("bot: reply failed", "room", roomID, "err", err)
 	}
 }
@@ -130,6 +137,10 @@ func (d *Dispatcher) run(ctx context.Context, cmd, service string, client arr.Cl
 	}
 
 	top := results[0]
+	if top.Exists {
+		msg := fmt.Sprintf("%s already has %s in the library.", service, formatTitle(top))
+		return msg, html.EscapeString(msg)
+	}
 	if err := client.Add(ctx, top); err != nil {
 		slog.Error("bot: add failed", "service", service, "title", top.Title, "err", err)
 		msg := fmt.Sprintf("%s: failed to add %s: %v", service, formatTitle(top), err)