feat: add session capacity waitlist

src/GmRelay.Bot/Migrations/V006__add_session_capacity_waitlist.sql

@@ -0,0 +1,16 @@
+-- Add per-session seat limits and participant waitlist support.
+ALTER TABLE sessions
+    ADD COLUMN max_players INTEGER,
+    ADD CONSTRAINT ck_sessions_max_players CHECK (max_players IS NULL OR max_players > 0);
+
+ALTER TABLE session_participants
+    ADD COLUMN registration_status VARCHAR(50) NOT NULL DEFAULT 'Active'
+        CHECK (registration_status IN ('Active', 'Waitlisted')),
+    ADD COLUMN created_at TIMESTAMPTZ NOT NULL DEFAULT now();
+
+CREATE INDEX ix_session_participants_session_registration_status
+    ON session_participants (session_id, registration_status);
+
+CREATE INDEX ix_session_participants_waitlist_order
+    ON session_participants (session_id, created_at, id)
+    WHERE registration_status = 'Waitlisted';