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

docker-compose.yml

@@ -19,6 +19,7 @@ services:
 
   app:
     build: .
+    restart: unless-stopped
     depends_on:
       db:
         condition: service_healthy