feat(settings): add canonical typed storage for the settings contract

The cross-platform settings contract needs one typed store behind it before a
resolver, routes or a migration can exist. This adds that storage to both
user-store backends and holds them to identical behavior.

PostgreSQL gets user_setting_values with the scope CHECK constraints, the five
partial unique indexes that enforce one explicit value per identity, and the
covering indexes the one-query read path needs, plus user_setting_mutations for
mutation_id idempotency and the inert user_setting_migration_rejects audit
table. The per-user SQLite store gets the same shape minus user_id, since that
database is already user-scoped.

The UserStore interface grows the typed operations: read one explicit value at
one scope, collect every candidate row for a resolution request in a single
query, upsert with a revision increment, unset, and the idempotency receipt
operations. The resolution read deliberately returns unranked candidates so the
resolver can rank in Go — one query per request, never one per scope, which the
pgx query-count test pins.

Delete behavior is application-enforced. Neither backend can inherit it from
constraints: the SQLite store declares no foreign keys, and library, series and
device columns are not FK targets in Postgres either. Profile deletion cascades
to profile-anchored values while account scope survives, forgetting a device
clears its profile_device values alongside the legacy overrides, and the
library/series purges remove only what is scoped to that entity.

The shared conformance suite covers all of it, including the set-versus-unset
distinction for false, 0, "" and null, so a divergence between the two backends
fails a test rather than reaching a client.

Part of #376

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

internal/userdb/sqlitestore.go

@@ -3,6 +3,7 @@ package userdb
 import (
 	"context"
 	"database/sql"
+	"encoding/json"
 	"fmt"
 	"strconv"
 	"time"
@@ -420,3 +421,49 @@ func (s *SQLiteUserStore) UpsertLibraryPlaybackPreference(_ context.Context, pre
 func (s *SQLiteUserStore) DeleteLibraryPlaybackPreference(_ context.Context, profileID string, libraryID int) error {
 	return DeleteLibraryPlaybackPreference(s.db, profileID, libraryID)
 }
+
+// --- Canonical typed setting values ---
+
+func (s *SQLiteUserStore) GetSettingValue(_ context.Context, id userstore.SettingIdentity) (*userstore.SettingValue, error) {
+	return GetSettingValue(s.db, id)
+}
+
+func (s *SQLiteUserStore) ListSettingValuesForResolution(_ context.Context, query userstore.SettingResolutionQuery) ([]userstore.SettingValue, error) {
+	return ListSettingValuesForResolution(s.db, query)
+}
+
+func (s *SQLiteUserStore) UpsertSettingValue(_ context.Context, id userstore.SettingIdentity, value json.RawMessage) (*userstore.SettingValue, error) {
+	return UpsertSettingValue(s.db, id, value)
+}
+
+func (s *SQLiteUserStore) DeleteSettingValue(_ context.Context, id userstore.SettingIdentity) (bool, error) {
+	return DeleteSettingValue(s.db, id)
+}
+
+func (s *SQLiteUserStore) DeleteSettingValuesForProfile(_ context.Context, profileID string) (int64, error) {
+	return DeleteSettingValuesForProfile(s.db, profileID)
+}
+
+func (s *SQLiteUserStore) DeleteSettingValuesForDevice(_ context.Context, profileID, deviceID string) (int64, error) {
+	return DeleteSettingValuesForDevice(s.db, profileID, deviceID)
+}
+
+func (s *SQLiteUserStore) DeleteSettingValuesForLibrary(_ context.Context, libraryID int) (int64, error) {
+	return DeleteSettingValuesForLibrary(s.db, libraryID)
+}
+
+func (s *SQLiteUserStore) DeleteSettingValuesForSeries(_ context.Context, seriesID string) (int64, error) {
+	return DeleteSettingValuesForSeries(s.db, seriesID)
+}
+
+func (s *SQLiteUserStore) GetSettingMutation(_ context.Context, mutationID string) (*userstore.SettingMutationRecord, error) {
+	return GetSettingMutation(s.db, mutationID)
+}
+
+func (s *SQLiteUserStore) PutSettingMutation(_ context.Context, record userstore.SettingMutationRecord) (userstore.SettingMutationRecord, bool, error) {
+	return PutSettingMutation(s.db, record)
+}
+
+func (s *SQLiteUserStore) DeleteExpiredSettingMutations(_ context.Context, before time.Time) (int64, error) {
+	return DeleteExpiredSettingMutations(s.db, before)
+}