feat(persistence): local-first story persistence (IndexedDB + Supabase skeleton)

Remove Cloudflare D1 entirely (4 API routes, lib/db/, Drizzle config/migrations,
drizzle-orm/drizzle-kit deps, wrangler D1/R2/KV bindings) and replace with
browser-local-first architecture:

Open-source build (IndexedDB, no auth):
- lib/persistence/ 5-file module: types, idb adapter (zero-dep, fault-tolerant,
  post-open invalidation retry), localStore (CRUD + sync-reserved metadata +
  slim/rebuild + retention-cap eviction with tombstone reap + sync-state
  protection + last-resort bounded fallback), sessionSlim (voice strip +
  styleRef absent-delete), cloudStore (Supabase skeleton, server-only)
- Autosave: persistence fingerprint (history.length:lastBeatCount:playerName),
  serial saveChain, failure rollback retry, replaySourceRef guard (prevents
  replayed shared stories from clobbering user saves)
- clientStoryPersistence.ts: thin facade (SaveResult discriminated union)
- Stories page: /[locale]/stories with 3-language i18n (zh-CN/en/ja)
- Homepage: book icon entry point in header

Commercial build (Supabase, skeleton only):
- Single table public.stories (JSONB + RLS 4 policies on auth.uid()=user_id)
- supabase/migrations/ DDL (idempotent)
- cloudStore.ts server-only repository, AUTH_ENABLED short-circuit
- Not wired to client this phase

Featured stories: pure fallback (buildFallbackCards + localizeCards), no D1

Includes fixes from 3 rounds of subagent code-review (tasks 16-30):
- CR1: autosave restructure, coerceOrientation, D1 comment cleanup
- CR2: fingerprint+serial+rollback+replay guard, idb post-open retry,
  enforceRetentionCap latent defense, sessionSlim absent invariant
- CR3: single-scene share guard (replaySourceRef), insert-beat fingerprint
  (beats.length), pass3 overflow double-count fix, detach gate unification

lib/persistence/types.ts

@@ -0,0 +1,98 @@
+// Persistence wire types — local-first story storage.
+//
+// Shared shapes for the browser-local store (IndexedDB) and the future Supabase
+// cloud store. Replaces the deleted D1 `lib/db/repositories/storyRepo` types,
+// severing all dependency on Drizzle / D1. The local `StoryRecord` and the cloud
+// `public.stories` row both carry the same slim `Session` blob (see
+// `SlimStoryBlob`) so there is no dual data shape to reconcile when cloud sync
+// is layered on next phase.
+
+import type { Session, Orientation } from "@infiplot/types";
+
+/** Schema version stamped on every local record — migration hook for future
+ *  structural evolution of `StoryRecord`. Bump when the on-disk shape changes. */
+export const STORY_SCHEMA_VERSION = 1;
+
+/** Coerce a Date | string | number (or anything) to epoch milliseconds, falling
+ *  back when the value is unparseable. Shared by the local store, the cloud store
+ *  (Supabase timestamptz), and the stories list UI — every site where a timestamp
+ *  crosses a storage/serialization boundary and could arrive as a non-number,
+ *  guarding against the historical `t.getTime is not a function` white-screen. */
+export function coerceEpoch(value: unknown, fallback: number): number {
+  if (typeof value === "number" && !Number.isNaN(value)) return value;
+  const d = value instanceof Date ? value : new Date(value as string | number);
+  const t = d.getTime();
+  return Number.isNaN(t) ? fallback : t;
+}
+
+/** local-first sync state of a record.
+ *  - "local-only": never sent to the cloud (open-source default, or pre-sync).
+ *  - "synced":     in agreement with the cloud row.
+ *  - "pending":    has un-propagated local changes (incl. soft-delete tombstones). */
+export type SyncState = "local-only" | "synced" | "pending";
+
+/** List-view projection of a saved story — the lightweight metadata the
+ *  "我的剧情" page renders without parsing the full session blob. Migrated out of
+ *  the deleted D1 `storyRepo`; timestamps are unified to epoch milliseconds
+ *  (the old D1 shape used `Date` and carried `userId`/`status`, both dropped:
+ *  the local layer has no account concept, and `status` was a D1 leftover). */
+export type StoryMeta = {
+  id: string;
+  worldSetting: string;
+  styleGuide: string;
+  orientation: Orientation;
+  sceneCount: number;
+  /** epoch ms */
+  createdAt: number;
+  /** epoch ms */
+  updatedAt: number;
+};
+
+/** The shared core payload for one saved story, identical between the local
+ *  record and the (future) cloud row. `session` is the SLIM `Session` — the
+ *  bulky reconstructible fields (`voice.referenceAudioBase64`,
+ *  `styleReferenceImage`) are stripped before persistence by the store layer. */
+export type SlimStoryBlob = {
+  id: string;
+  worldSetting: string;
+  styleGuide: string;
+  orientation: Orientation;
+  sceneCount: number;
+  rev: number;
+  /** Slim Session (voice + styleReferenceImage stripped). Type stays `Session`;
+   *  slimming is a runtime guarantee enforced by the store, not the type. */
+  session: Session;
+};
+
+/** One row in the browser-local IndexedDB store (object store keyPath = "id").
+ *  Carries the slim session payload plus the local-first sync-reserved
+ *  metadata so cloud sync can be layered on next phase without restructuring. */
+export type StoryRecord = {
+  id: string;
+  /** = STORY_SCHEMA_VERSION at write time. */
+  schemaVersion: number;
+
+  // ── List-view metadata (denormalized so listing needn't parse the blob) ──
+  worldSetting: string;
+  styleGuide: string;
+  orientation: Orientation;
+  sceneCount: number;
+
+  // ── local-first sync-reserved fields ──
+  /** epoch ms; set on first save, preserved across subsequent upserts. */
+  createdAt: number;
+  /** epoch ms; refreshed on every save. */
+  updatedAt: number;
+  /** Revision counter; new record = 1, bumped on each local save. */
+  rev: number;
+  /** Soft-delete tombstone (epoch ms) or null. Delete sets this rather than
+   *  physically removing the row, so the deletion can propagate to the cloud
+   *  next phase. List queries filter tombstoned records out. */
+  deletedAt: number | null;
+  syncState: SyncState;
+
+  // ── Payload ──
+  /** Slim Session (voice + styleReferenceImage stripped). IndexedDB
+   *  structured-clones objects, so this is stored as-is (no JSON.stringify). */
+  session: Session;
+};