fix(03): correct event-write timezone + per-user calendar identity (Gate 2 Part D)

BUG A — timed events written 4h off: EventForm sent a naive local wall-clock
string with no offset; the UTC API container parsed it via new Date() as UTC, so
09:00 America/Toronto serialized to DTSTART:...090000Z. Fix: new
apps/pwa/src/lib/eventDateTime.ts serializes timed events to an unambiguous UTC
instant in the browser (where the operator's zone is known); all-day stays a DATE
string. No backend change.

BUG B — created events attached to the wrong user's calendar + duplicate calendar
rows per poll: calendars had no unique key on url, and poller/sync matched
calendars by url alone — so under the shared single Fastmail account (D-16) one
member's collection resolved to the other member's row. Fix: composite
unique(user_id, url); scope poller lookup + sync select to (userId, url); hand
migration 0001 (dedup + add key), applied to the live DB.

Regression tests fail against the buggy url-only predicate. API 98/98, PWA 140/140,
tsc clean both packages.

apps/api/src/db/schema.ts

@@ -73,7 +73,16 @@ export const calendars = mysqlTable(
     lastSyncedAt: timestamp('last_synced_at'),
     isShared: boolean('is_shared').default(false).notNull(),
   },
-  (t) => [index('idx_calendars_user_id').on(t.userId)],
+  (t) => [
+    index('idx_calendars_user_id').on(t.userId),
+    // BUG B: calendar identity is (userId, url), not url alone. The two household
+    // members share one Fastmail account (D-16), so the SAME collection URL is
+    // polled by both credentials. Without this unique key the calendar upsert's
+    // onDuplicateKeyUpdate never fired → a new row per poll, and the url-only
+    // lookup resolved to the other member's row → events cached under the wrong
+    // calendarId. Keying on (userId, url) makes the upsert idempotent per member.
+    unique('uniq_calendar_user_url').on(t.userId, t.url),
+  ],
 )
 
 /**