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/0030_enrollments.sql

@@ -0,0 +1,19 @@
+-- Phase 3: Enrollments — member enrollment in a schedule slot
+
+CREATE TYPE "enrollment_status" AS ENUM ('active', 'paused', 'cancelled', 'completed');
+
+CREATE TABLE "enrollment" (
+  "id" uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  "member_id" uuid NOT NULL REFERENCES "member"("id"),
+  "account_id" uuid NOT NULL REFERENCES "account"("id"),
+  "schedule_slot_id" uuid NOT NULL REFERENCES "schedule_slot"("id"),
+  "instructor_id" uuid NOT NULL REFERENCES "instructor"("id"),
+  "status" enrollment_status NOT NULL DEFAULT 'active',
+  "start_date" date NOT NULL,
+  "end_date" date,
+  "monthly_rate" numeric(10,2),
+  "makeup_credits" integer NOT NULL DEFAULT 0,
+  "notes" text,
+  "created_at" timestamptz NOT NULL DEFAULT now(),
+  "updated_at" timestamptz NOT NULL DEFAULT now()
+);

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
     }
   ]
 }