* feat(calendar): support multiple CalDAV accounts
Replaces the single CalDAV credential slot with a named account list so
users can sync both a personal and work calendar simultaneously.
- Add `account_id` column to `CalendarCal` + startup migration
- `_load_caldav_accounts()` in caldav_sync.py reads `caldav_accounts`
list from prefs, auto-migrating the legacy single `caldav` key on
first use (no user action required)
- `sync_caldav()` iterates all accounts and aggregates counts/errors
- `writeback_event()` resolves credentials via `CalendarCal.account_id`,
falling back to the first account for legacy rows
- New REST endpoints: GET/POST/PUT/DELETE `/api/calendar/config/accounts`
- Legacy GET/POST `/api/calendar/config` preserved for backward compat
- Settings UI: one card per account with Label, URL, Username, Password
fields; Test button works for both unsaved (inline creds) and saved
(by account_id) accounts; delete removes only that account
- Update test_caldav_url_hardening.py mock to include `_save_for_user`
and updated `_sync_blocking` signature
* fix(calendar): restore #2765 PK scoping and #2819 writeback URL validation
Two regressions introduced by the multi-account refactor:
1. PK collision (#2765): _stable_cal_id was back to hashing only the URL,
so two users — or one user with two accounts on the same server — would
collide on the primary key. Restore owner+account_id in the hash key
(format: "{owner}\n{account_id}\n{url}") and thread both values through
_sync_blocking → _writeback_blocking → push_event → find_remote_calendar
so the hash round-trips correctly on write-back.
2. URL validation dropped (#2819): _load_caldav_accounts imported
_save_for_user at function scope, causing an ImportError on test mocks
that only provide _load_for_user, which prevented writeback_event from
reaching the validate_caldav_url call. Move the import inside the
migration branch and wrap in try/except (best-effort save; next call
re-migrates from the still-present legacy key).
Update fake_writeback_blocking in test_caldav_writeback.py to accept the
new owner/account_id optional params.
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