fix: wait for postgres before running migrations on startup

App was crash-looping when the host restarted and Docker brought up the
app container before postgres was ready. Added a pg_isready poll loop in
entrypoint.sh and installed postgresql-client in the image.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

entrypoint.sh

@@ -1,4 +1,10 @@
 #!/bin/sh
 set -e
+
+until pg_isready -q -h postgres; do
+  echo "Waiting for postgres..."
+  sleep 2
+done
+
 alembic upgrade head
 exec uvicorn corvid.main:app --host 0.0.0.0 --port 8080