feat(db): add external_calendars table and link to calendar events (migration v14)

Stores display name and color per synced Google/Apple calendar.
calendar_events gains a calendar_ref_id FK for join-based name/color lookup.

server/db.js

@@ -523,6 +523,27 @@ const MIGRATIONS = [
       CREATE INDEX IF NOT EXISTS idx_meals_recipe_id ON meals(recipe_id);
     `,
   },
+  {
+    version: 14,
+    description: 'Externe Kalender-Metadaten (Name, Farbe) und Verknüpfung mit Events',
+    up: `
+      CREATE TABLE IF NOT EXISTS external_calendars (
+        id          INTEGER PRIMARY KEY AUTOINCREMENT,
+        source      TEXT    NOT NULL CHECK(source IN ('google', 'apple')),
+        external_id TEXT    NOT NULL,
+        name        TEXT    NOT NULL,
+        color       TEXT,
+        UNIQUE(source, external_id)
+      );
+
+      CREATE INDEX IF NOT EXISTS idx_ext_cal_source ON external_calendars(source, external_id);
+
+      ALTER TABLE calendar_events ADD COLUMN calendar_ref_id INTEGER
+        REFERENCES external_calendars(id) ON DELETE SET NULL;
+
+      CREATE INDEX IF NOT EXISTS idx_cal_events_ref ON calendar_events(calendar_ref_id);
+    `,
+  },
 ];
 
 /**