Add optional Authentik (OIDC) sign-in with server-side preference sync

Signed-in users get their preferences (hidden feeds, weather location,
weather toggle) stored server-side keyed by their OIDC subject and synced
across devices. Anonymous visitors keep using browser localStorage, so the
site stays public. First sign-in migrates existing localStorage prefs up.

- config: [web.auth] section (issuer, client_id/secret, redirect, session_secret)
- storage: user_preferences table + Get/PutUserPrefs
- web/auth: OIDC code flow, HMAC-signed session cookie, CSRF state + nonce
- web/prefs_api: GET/PUT /api/preferences (auth-gated, 64KB cap)
- frontend: prefs.js sync layer seeds localStorage from server, pushes on write
- header: sign-in / account control

OIDC discovery is non-fatal at boot: if Authentik is down, Pete serves
anonymously rather than refusing to start.

internal/storage/prefs.go

@@ -0,0 +1,50 @@
+package storage
+
+import (
+	"database/sql"
+	"errors"
+	"fmt"
+)
+
+// UserPrefs is one signed-in user's stored state. Prefs is an opaque JSON blob
+// owned by the web frontend (disabled feeds, weather location, toggles, …) —
+// the backend just persists and returns it, keyed by the OIDC subject.
+type UserPrefs struct {
+	Sub       string
+	Prefs     string
+	Username  string
+	Email     string
+	UpdatedAt int64
+}
+
+// GetUserPrefs returns the stored prefs blob for an OIDC subject. It returns
+// ("", nil) when the user has no record yet (first sign-in).
+func GetUserPrefs(sub string) (string, error) {
+	var prefs string
+	err := Get().QueryRow(`SELECT prefs FROM user_preferences WHERE user_sub = ?`, sub).Scan(&prefs)
+	if errors.Is(err, sql.ErrNoRows) {
+		return "", nil
+	}
+	if err != nil {
+		return "", fmt.Errorf("get user prefs: %w", err)
+	}
+	return prefs, nil
+}
+
+// PutUserPrefs upserts a user's prefs blob along with identity hints (username,
+// email) for later features. The prefs string is stored verbatim.
+func PutUserPrefs(sub, prefs, username, email string) error {
+	_, err := Get().Exec(`
+		INSERT INTO user_preferences (user_sub, prefs, username, email, updated_at)
+		VALUES (?, ?, ?, ?, ?)
+		ON CONFLICT(user_sub) DO UPDATE SET
+			prefs      = excluded.prefs,
+			username   = excluded.username,
+			email      = excluded.email,
+			updated_at = excluded.updated_at`,
+		sub, prefs, username, email, nowUnix())
+	if err != nil {
+		return fmt.Errorf("put user prefs: %w", err)
+	}
+	return nil
+}

internal/storage/prefs_test.go

@@ -0,0 +1,44 @@
+package storage
+
+import (
+	"path/filepath"
+	"testing"
+)
+
+func TestUserPrefsRoundTrip(t *testing.T) {
+	if err := Init(filepath.Join(t.TempDir(), "prefs.db")); err != nil {
+		t.Fatalf("init db: %v", err)
+	}
+	t.Cleanup(func() { _ = Close() })
+
+	// Missing user → empty, no error.
+	got, err := GetUserPrefs("nobody")
+	if err != nil {
+		t.Fatalf("get missing: %v", err)
+	}
+	if got != "" {
+		t.Fatalf("expected empty for missing user, got %q", got)
+	}
+
+	blob := `{"pete.weather.loc.v1":"{\"postal\":\"1000\"}"}`
+	if err := PutUserPrefs("ak-sub-1", blob, "misaki", "m@example.com"); err != nil {
+		t.Fatalf("put: %v", err)
+	}
+	got, err = GetUserPrefs("ak-sub-1")
+	if err != nil {
+		t.Fatalf("get: %v", err)
+	}
+	if got != blob {
+		t.Fatalf("blob mismatch:\n got %q\nwant %q", got, blob)
+	}
+
+	// Upsert: second put for same sub replaces the blob.
+	blob2 := `{"pete-weather-off":"1"}`
+	if err := PutUserPrefs("ak-sub-1", blob2, "misaki", "m@example.com"); err != nil {
+		t.Fatalf("upsert: %v", err)
+	}
+	got, _ = GetUserPrefs("ak-sub-1")
+	if got != blob2 {
+		t.Fatalf("upsert mismatch: got %q want %q", got, blob2)
+	}
+}

internal/storage/schema.go

@@ -45,6 +45,14 @@ CREATE TABLE IF NOT EXISTS reactions (
     reacted_at INTEGER NOT NULL
 );
 
+CREATE TABLE IF NOT EXISTS user_preferences (
+    user_sub   TEXT PRIMARY KEY,
+    prefs      TEXT NOT NULL,
+    username   TEXT,
+    email      TEXT,
+    updated_at INTEGER NOT NULL
+);
+
 CREATE UNIQUE INDEX IF NOT EXISTS idx_post_log_guid_channel ON post_log(guid, channel);
 CREATE INDEX IF NOT EXISTS idx_post_log_event_id ON post_log(event_id);
 CREATE INDEX IF NOT EXISTS idx_post_log_channel_posted ON post_log(channel, posted_at);