ops: wait for the database on startup (no crash-loop on a slow DB)

New thoughtsync.dbwait polls the DB (SELECT 1) up to 60×1s before startup,
logging each attempt, so a briefly slow/unready database no longer crash-
loops the container. Wired as `python -m thoughtsync.dbwait &&` ahead of
`alembic upgrade head` in the image CMD and the dev compose command; exits
non-zero after the window so a restart policy can take over. Prod compose
gains restart: unless-stopped as the complementary piece.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

src/thoughtsync/dbwait.py

@@ -0,0 +1,54 @@
+from __future__ import annotations
+
+import asyncio
+import sys
+
+from sqlalchemy import text
+from sqlalchemy.ext.asyncio import create_async_engine
+
+from .config import Config
+
+ATTEMPTS = 60
+DELAY_SECONDS = 1.0
+
+
+async def _probe(url: str) -> None:
+    engine = create_async_engine(url)
+    try:
+        async with engine.connect() as conn:
+            await conn.execute(text("SELECT 1"))
+    finally:
+        await engine.dispose()
+
+
+async def wait_for_db(attempts: int = ATTEMPTS, delay: float = DELAY_SECONDS) -> bool:
+    """Poll the database until it accepts a connection, up to `attempts` tries.
+
+    Startup runs `alembic upgrade head` as the container's first DB touch, so a
+    database that is a moment slow to accept connections would otherwise crash the
+    container. This gives it a short window to come up, verifying once per `delay`
+    seconds, instead of failing on the first missed connection.
+    """
+    url = Config.DATABASE_URL
+    for attempt in range(1, attempts + 1):
+        try:
+            await _probe(url)
+            print(f"[dbwait] database ready (attempt {attempt}/{attempts})", flush=True)
+            return True
+        except Exception as exc:
+            print(
+                f"[dbwait] not ready (attempt {attempt}/{attempts}): {exc.__class__.__name__}: {exc}",
+                flush=True,
+            )
+            if attempt < attempts:
+                await asyncio.sleep(delay)
+    print(f"[dbwait] database unreachable after {attempts} attempts; giving up", flush=True)
+    return False
+
+
+def main() -> int:
+    return 0 if asyncio.run(wait_for_db()) else 1
+
+
+if __name__ == "__main__":
+    sys.exit(main())