server: reconcile ai_sessions persistence to Chunk 9 AISession shape

Reshape the ai_sessions table (migration 003) and mapping/routes to the
full AISession contract: title, kind, notes, created_at, last_message_at;
drop legacy role/model/settings. toAISession now maps the new columns and
routes create/patch/messages handlers use the new shape.

server/src/migrations/003_ai_sessions.sql

@@ -0,0 +1,24 @@
+-- Chunk #9: reshape ai_sessions to the full AISession shape.
+-- Adds title, kind, notes, created_at, last_message_at; drops role, model, settings.
+CREATE TABLE ai_sessions_new (
+  id          TEXT PRIMARY KEY,
+  project_id  TEXT NOT NULL REFERENCES projects(id) ON DELETE CASCADE,
+  title       TEXT NOT NULL DEFAULT '',
+  kind        TEXT NOT NULL DEFAULT 'session',
+  messages    TEXT NOT NULL DEFAULT '[]',
+  suggestions TEXT NOT NULL DEFAULT '[]',
+  notes       TEXT NOT NULL DEFAULT '[]',
+  created_at  TEXT NOT NULL,
+  updated_at  TEXT NOT NULL,
+  last_message_at TEXT
+);
+
+INSERT INTO ai_sessions_new (id, project_id, title, kind, messages, suggestions, notes, created_at, updated_at, last_message_at)
+SELECT id, project_id, '', 'session', messages, suggestions, '[]', updated_at, updated_at, NULL
+FROM ai_sessions;
+
+DROP TABLE ai_sessions;
+
+ALTER TABLE ai_sessions_new RENAME TO ai_sessions;
+
+CREATE INDEX IF NOT EXISTS idx_ai_sessions_project ON ai_sessions(project_id);