feat: add SQLite database layer with schema init

src/backend/db/database.py

@@ -0,0 +1,61 @@
+import uuid
+from datetime import datetime, timezone
+
+import aiosqlite
+
+from src.backend.config import settings
+
+SCHEMA = """
+CREATE TABLE IF NOT EXISTS topics (
+    id TEXT PRIMARY KEY,
+    name TEXT NOT NULL UNIQUE,
+    system_prompt TEXT NOT NULL,
+    summary TEXT DEFAULT '',
+    model TEXT,
+    created_at TEXT NOT NULL,
+    updated_at TEXT NOT NULL
+);
+
+CREATE TABLE IF NOT EXISTS requests (
+    id TEXT PRIMARY KEY,
+    session_id TEXT,
+    topic_id TEXT NOT NULL REFERENCES topics(id),
+    prompt TEXT NOT NULL,
+    model TEXT,
+    summarize INTEGER DEFAULT 0,
+    status TEXT NOT NULL DEFAULT 'pending',
+    result TEXT,
+    error TEXT,
+    created_at TEXT NOT NULL,
+    completed_at TEXT
+);
+
+CREATE INDEX IF NOT EXISTS idx_requests_status ON requests(status);
+CREATE INDEX IF NOT EXISTS idx_requests_topic_id ON requests(topic_id);
+CREATE INDEX IF NOT EXISTS idx_requests_session_id ON requests(session_id);
+"""
+
+
+def _now() -> str:
+    return datetime.now(timezone.utc).isoformat()
+
+
+def _new_id() -> str:
+    return str(uuid.uuid4())
+
+
+async def get_db() -> aiosqlite.Connection:
+    db = await aiosqlite.connect(settings.db_path)
+    db.row_factory = aiosqlite.Row
+    await db.execute("PRAGMA journal_mode=WAL")
+    await db.execute("PRAGMA foreign_keys=ON")
+    return db
+
+
+async def init_db():
+    db = await get_db()
+    try:
+        await db.executescript(SCHEMA)
+        await db.commit()
+    finally:
+        await db.close()