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.

lib/persistence/types.ts

@@ -19,7 +19,9 @@ export const STORY_SCHEMA_VERSION = 1;
  *  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;
+  // Number.isFinite (not just !isNaN) so ±Infinity also falls through to the
+  // fallback — new Date(Infinity).getTime() is NaN, not a usable epoch.
+  if (typeof value === "number" && Number.isFinite(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;