feat(calendar): support multiple CalDAV accounts (#2942)

* 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.

core/database.py

@@ -1458,7 +1458,11 @@ class CalendarCal(TimestampMixin, Base):
     owner = Column(String, nullable=True, index=True)
     name  = Column(String, nullable=False)
     color = Column(String, default="#5b8abf")
-    source = Column(String, default="local")  # "local" or "timetree"
+    source = Column(String, default="local")  # "local" or "caldav"
+    # UUID of the CalDAV account in user prefs that owns this calendar.
+    # NULL for local calendars and for CalDAV calendars created before
+    # multi-account support was added (treated as "use any configured account").
+    account_id = Column(String, nullable=True, index=True)
 
     events = relationship("CalendarEvent", back_populates="calendar", cascade="all, delete-orphan")
 
@@ -1622,6 +1626,7 @@ def init_db():
     _migrate_add_calendar_metadata()
     _migrate_add_calendar_is_utc()
     _migrate_add_calendar_origin()
+    _migrate_add_calendar_account_id()
     _migrate_encrypt_email_passwords()
     _migrate_encrypt_signatures()
     _migrate_encrypt_endpoint_keys()
@@ -1786,6 +1791,27 @@ def _migrate_add_calendar_origin():
         logging.getLogger(__name__).warning(f"calendar_events.origin migration failed: {e}")
 
 
+def _migrate_add_calendar_account_id():
+    """Add `account_id` to calendars so each CalDAV-backed calendar knows which
+    credential set (from caldav_accounts in user prefs) owns it. Idempotent."""
+    import sqlite3
+    db_path = DATABASE_URL.replace("sqlite:///", "")
+    if not os.path.exists(db_path):
+        return
+    try:
+        conn = sqlite3.connect(db_path)
+        cursor = conn.execute("PRAGMA table_info(calendars)")
+        columns = [row[1] for row in cursor.fetchall()]
+        if columns and "account_id" not in columns:
+            conn.execute("ALTER TABLE calendars ADD COLUMN account_id TEXT")
+            conn.execute("CREATE INDEX IF NOT EXISTS ix_calendars_account_id ON calendars(account_id)")
+            conn.commit()
+            logging.getLogger(__name__).info("Migrated: added 'account_id' column to calendars")
+        conn.close()
+    except Exception as e:
+        logging.getLogger(__name__).warning(f"calendars.account_id migration failed: {e}")
+
+
 def _migrate_add_calendar_metadata():
     """Add importance/event_type/last_pinged columns to calendar_events table."""
     import sqlite3