Add lessons Phase 3: enrollments with capacity and time conflict checks

Links members to schedule slots via enrollments. Enforces max_students
capacity on slots and prevents members from double-booking the same
day/time. Supports status transitions and filtering. 11 new tests
(51 total lessons tests).

packages/backend/src/db/migrations/meta/_journal.json

@@ -211,6 +211,13 @@
       "when": 1774890000000,
       "tag": "0029_schedule_slots",
       "breakpoints": true
+    },
+    {
+      "idx": 30,
+      "version": "7",
+      "when": 1774900000000,
+      "tag": "0030_enrollments",
+      "breakpoints": true
     }
   ]
 }