feat(classes): weekly recurring schedules with lazy session materialization

class_schedules rules generate concrete session rows for a rolling 28-day
horizon — no cron; materialize runs idempotently on dashboard and public
reads via a partial unique index on (schedule_id, starts_at). Rule delete
stops future sessions; existing sessions keep bookings and overrides.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/backend/migrations/00018_class_schedules.sql

@@ -0,0 +1,27 @@
+-- +goose Up
+-- Weekly recurring rules for class sessions. Rules materialize concrete
+-- session rows for a rolling horizon (the service writes them); sessions
+-- keep a nullable schedule_id link so regeneration is idempotent via the
+-- partial unique index and deleting a rule stops future sessions without
+-- touching existing ones.
+CREATE TABLE class_schedules (
+  id          uuid PRIMARY KEY DEFAULT gen_random_uuid(),
+  tenant_id   uuid NOT NULL REFERENCES tenants(id) ON DELETE CASCADE,
+  template_id uuid NOT NULL REFERENCES class_templates(id) ON DELETE CASCADE,
+  location_id uuid REFERENCES locations(id) ON DELETE SET NULL,
+  day_of_week integer NOT NULL CHECK (day_of_week BETWEEN 0 AND 6),
+  starts_local time NOT NULL,
+  active      boolean NOT NULL DEFAULT true,
+  created_at  timestamptz NOT NULL DEFAULT now()
+);
+CREATE INDEX class_schedules_tenant_idx ON class_schedules (tenant_id);
+
+ALTER TABLE class_sessions
+  ADD COLUMN schedule_id uuid REFERENCES class_schedules(id) ON DELETE SET NULL;
+CREATE UNIQUE INDEX class_sessions_schedule_start_idx
+  ON class_sessions (schedule_id, starts_at)
+  WHERE schedule_id IS NOT NULL;
+
+-- +goose Down
+ALTER TABLE class_sessions DROP COLUMN IF EXISTS schedule_id;
+DROP TABLE IF EXISTS class_schedules;