feat: per-user volume leveling preference, synced across devices (M464 #4998)

Mode (off, auto, track, album), target (-18, -16, -14 LUFS) and boost
(within headroom, or fully with a limiter), stored per user on the server
so the web player, the Android app and casts apply the same one.

- Server: user_normalization_prefs (migration 0067), GET/PUT
  /api/me/normalization; a whole-body PUT, validated, last write wins.
- Web: Settings > Playback > Volume leveling. Saves at once, restores the
  old choice if the save fails, and caches the value for the player.
- Android: Settings card. The device keeps a copy for offline playback
  (Room v9 with an explicit migration, so the upgrade wipes nothing).
  Writes are offline-first: shown at once, PUT best effort, queued on
  failure (NORMALIZATION_SET, collapsed to the newest). A refresh never
  overwrites a change still queued.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/me_normalization.go

@@ -0,0 +1,51 @@
+package api
+
+import (
+	"encoding/json"
+	"errors"
+	"net/http"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/apierror"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/library"
+)
+
+// GET /api/me/normalization returns the caller's loudness-normalization
+// preference, or the defaults when they have never set one (M464 #4998).
+func (h *handlers) handleGetMyNormalization(w http.ResponseWriter, r *http.Request) {
+	caller, ok := requireUser(w, r)
+	if !ok {
+		return
+	}
+	p, err := library.LoadNormalizationPrefs(r.Context(), dbq.New(h.pool), caller.ID)
+	if err != nil {
+		writeErrWithLog(w, h.logger, "normalization: load", apierror.InternalMsg("lookup failed", err))
+		return
+	}
+	writeJSON(w, http.StatusOK, p)
+}
+
+// PUT /api/me/normalization replaces the caller's preference with the whole
+// body. Every field is required, so a client replaying a queued change
+// offline sends the full state it showed the user, and the last write wins.
+func (h *handlers) handlePutMyNormalization(w http.ResponseWriter, r *http.Request) {
+	caller, ok := requireUser(w, r)
+	if !ok {
+		return
+	}
+	var body library.NormalizationPrefs
+	if err := json.NewDecoder(r.Body).Decode(&body); err != nil {
+		writeErr(w, apierror.BadRequest("bad_body", "invalid JSON body"))
+		return
+	}
+	p, err := library.SaveNormalizationPrefs(r.Context(), dbq.New(h.pool), caller.ID, body)
+	if errors.Is(err, library.ErrNormalizationPrefsInvalid) {
+		writeErr(w, apierror.BadRequest("invalid_normalization", err.Error()))
+		return
+	}
+	if err != nil {
+		writeErrWithLog(w, h.logger, "normalization: save", apierror.InternalMsg("save failed", err))
+		return
+	}
+	writeJSON(w, http.StatusOK, p)
+}