fix(coder): CASCADE delete on session_worktrees + agent_sessions FKs

Deleting a session with linked session_worktrees or agent_sessions rows
threw a FK violation (500 on DELETE /api/sessions/:id). Both FKs now
ON DELETE CASCADE. Idempotent migration: drops the old constraint and
re-adds with CASCADE only if confdeltype != 'c'.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/coder/src/schema.sql

@@ -78,15 +78,27 @@ ALTER TABLE tasks ADD COLUMN IF NOT EXISTS feature_values JSONB;
 
 -- v2.6: one shared worktree per session (all agents/panes in the session operate in it).
 CREATE TABLE IF NOT EXISTS session_worktrees (
-  session_id UUID PRIMARY KEY REFERENCES sessions(id),
+  session_id UUID PRIMARY KEY REFERENCES sessions(id) ON DELETE CASCADE,
   worktree_path TEXT NOT NULL,
   base_commit TEXT,
   created_at TIMESTAMPTZ NOT NULL DEFAULT clock_timestamp()
 );
+-- Migrate existing FK to CASCADE (idempotent: drops the old constraint if present).
+DO $$ BEGIN
+  IF EXISTS (
+    SELECT 1 FROM pg_constraint
+    WHERE conname = 'session_worktrees_session_id_fkey'
+      AND confdeltype <> 'c'
+  ) THEN
+    ALTER TABLE session_worktrees DROP CONSTRAINT session_worktrees_session_id_fkey;
+    ALTER TABLE session_worktrees ADD CONSTRAINT session_worktrees_session_id_fkey
+      FOREIGN KEY (session_id) REFERENCES sessions(id) ON DELETE CASCADE;
+  END IF;
+END $$;
 
 -- v2.6: one backend session per (session, agent); resumed on switch-back.
 CREATE TABLE IF NOT EXISTS agent_sessions (
-  session_id UUID NOT NULL REFERENCES sessions(id),
+  session_id UUID NOT NULL REFERENCES sessions(id) ON DELETE CASCADE,
   agent TEXT NOT NULL,
   backend TEXT NOT NULL,
   agent_session_id TEXT,
@@ -99,6 +111,19 @@ CREATE TABLE IF NOT EXISTS agent_sessions (
   CONSTRAINT agent_sessions_status_chk CHECK (status IN ('idle', 'active', 'crashed', 'closed'))
 );
 
+-- Migrate existing agent_sessions FK to CASCADE.
+DO $$ BEGIN
+  IF EXISTS (
+    SELECT 1 FROM pg_constraint
+    WHERE conname = 'agent_sessions_session_id_fkey'
+      AND confdeltype <> 'c'
+  ) THEN
+    ALTER TABLE agent_sessions DROP CONSTRAINT agent_sessions_session_id_fkey;
+    ALTER TABLE agent_sessions ADD CONSTRAINT agent_sessions_session_id_fkey
+      FOREIGN KEY (session_id) REFERENCES sessions(id) ON DELETE CASCADE;
+  END IF;
+END $$;
+
 -- v2.6: config fingerprint for stale-session detection (auto-recover on model change).
 ALTER TABLE agent_sessions ADD COLUMN IF NOT EXISTS config_hash TEXT;