fix(persistence): address PR review feedback (4 low-cost improvements)

From PR #114 external review agent — adopted the real, low-cost findings;
remaining items (false positives / design trade-offs) explained in PR replies:

- coerceEpoch: !Number.isNaN → Number.isFinite — reject ±Infinity, which
  previously slipped through and produced Invalid Date via new Date(Infinity)
- enforceRetentionCap pass2/pass3: decrement overflow only when idbDelete
  actually succeeds — a failed best-effort delete no longer under-evicts
- cloudListStories: explicit column list instead of select() — avoid pulling
  the bulky session_jsonb when only metadata is needed
- Supabase stories: composite primary key (user_id, id) + onConflict user_id,id
  — avoid a cross-user Session.id collision rejecting the second user's save
  (skeleton not yet deployed, so the migration is edited in place)

typecheck + build:cf green.

supabase/migrations/20260624135618_stories.sql

@@ -16,7 +16,7 @@
 -- dropped-then-created (Postgres has no `create policy if not exists`).
 
 create table if not exists public.stories (
-  id            text primary key,                                  -- = Session.id ("s_xxx"), shared with the local record
+  id            text not null,                                     -- = Session.id ("s_xxx"), unique only per user
   user_id       uuid not null references auth.users (id) on delete cascade,
   world_setting text not null default '',
   style_guide   text not null default '',
@@ -26,7 +26,11 @@ create table if not exists public.stories (
   created_at    timestamptz not null default now(),
   updated_at    timestamptz not null default now(),
   deleted_at    timestamptz,                                       -- soft-delete tombstone; null = live
-  session_jsonb jsonb not null                                     -- slim Session blob (voice + styleReferenceImage stripped)
+  session_jsonb jsonb not null,                                    -- slim Session blob (voice + styleReferenceImage stripped)
+  -- Composite PK: a random Session.id ("s_xxx") is unique only within a user, so
+  -- scope the key by user_id — otherwise a cross-user id collision would reject
+  -- the second user's save with a PK violation.
+  primary key (user_id, id)
 );
 
 -- List query path: a user's stories newest-first.