feat(db): add chat_turns idempotency table

Co-authored-by: Cursor <cursoragent@cursor.com>

deploy/postgres/context_init.sql

@@ -121,6 +121,29 @@ CREATE INDEX IF NOT EXISTS idx_chat_messages_sid_category ON chat_messages(sessi
 CREATE EXTENSION IF NOT EXISTS pg_trgm;
 CREATE INDEX IF NOT EXISTS idx_chat_messages_content_trgm ON chat_messages USING GIN (content gin_trgm_ops);
 
+CREATE TABLE IF NOT EXISTS chat_turns (
+    id                      BIGSERIAL PRIMARY KEY,
+    session_id              TEXT NOT NULL,
+    client_turn_id          TEXT NOT NULL,
+    user_id                 BIGINT,
+    project_id              BIGINT,
+    user_message_id         BIGINT REFERENCES chat_messages(id) ON DELETE SET NULL,
+    assistant_message_id    BIGINT REFERENCES chat_messages(id) ON DELETE SET NULL,
+    request_body_sha256     TEXT NOT NULL DEFAULT '',
+    status                  TEXT NOT NULL DEFAULT 'accepted',
+    owner_token             TEXT NOT NULL DEFAULT '',
+    lease_expires_at        TIMESTAMPTZ,
+    first_request_id        TEXT NOT NULL DEFAULT '',
+    last_request_id         TEXT NOT NULL DEFAULT '',
+    attempt_count           INT NOT NULL DEFAULT 1,
+    created_at              TIMESTAMPTZ NOT NULL DEFAULT now(),
+    updated_at              TIMESTAMPTZ NOT NULL DEFAULT now(),
+    completed_at            TIMESTAMPTZ,
+    CONSTRAINT uq_chat_turns_session_client UNIQUE (session_id, client_turn_id)
+);
+CREATE INDEX IF NOT EXISTS idx_chat_turns_session_status ON chat_turns(session_id, status);
+CREATE INDEX IF NOT EXISTS idx_chat_turns_client ON chat_turns(client_turn_id);
+
 -- 4.1 每日智能摘要表 (Daily Digests)
 CREATE TABLE IF NOT EXISTS chat_daily_digests (
     id               BIGSERIAL PRIMARY KEY,