fix: ensure data persistence across reinstallations

- Add entrypoint.sh: creates /app/data, stamps legacy DBs, runs alembic upgrade head
- Update Dockerfile to use new entrypoint and pre-create /app/data
- Change default DATABASE_URL to sqlite:///./data/dmarq.db
- Add _ensure_sqlite_dir() to database.py for automatic directory creation
- Update docker-compose.yml with app_data named volume for /app/data
- Add 6 tests for _ensure_sqlite_dir and updated default URL

Agent-Logs-Url: https://github.com/christianlouis/dmarq/sessions/2bac317b-97a2-450b-84f1-3e316f7ef54c

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

backend/entrypoint.sh

@@ -0,0 +1,57 @@
+#!/bin/sh
+# entrypoint.sh – prepare the database and start the application.
+#
+# Responsibilities:
+#   1. Create the data directory so SQLite can write its file there.
+#   2. Ensure Alembic migration tracking is consistent (stamp existing
+#      databases that were created before Alembic was introduced).
+#   3. Apply all pending Alembic migrations (alembic upgrade head).
+#   4. Hand off to the real application process.
+
+set -e
+
+DATA_DIR="${DATA_DIR:-/app/data}"
+mkdir -p "$DATA_DIR"
+
+echo "==> Running database migrations …"
+
+# If the database already has tables but no alembic_version table (i.e. it
+# was bootstrapped via SQLAlchemy create_all before this entrypoint was
+# introduced), stamp it at the current head so that subsequent 'upgrade head'
+# calls are no-ops rather than errors.
+python - <<'PYEOF'
+import sys
+try:
+    from sqlalchemy import inspect, text
+    from app.core.database import engine
+
+    with engine.connect() as conn:
+        insp = inspect(conn)
+        tables = insp.get_table_names()
+
+    if "alembic_version" not in tables and tables:
+        import subprocess
+        print(
+            "WARNING: database has tables but no alembic_version – "
+            "stamping head to record current schema state."
+        )
+        result = subprocess.run(
+            ["alembic", "stamp", "head"],
+            capture_output=True,
+            text=True,
+        )
+        if result.returncode != 0:
+            print("ERROR: alembic stamp failed:", result.stderr, file=sys.stderr)
+            sys.exit(1)
+        print(result.stdout)
+except Exception as exc:  # noqa: BLE001
+    # If we cannot connect at all (e.g. Postgres not ready yet), let
+    # 'alembic upgrade head' handle the error with a clear message.
+    print(f"WARNING: pre-migration inspection skipped: {exc}")
+PYEOF
+
+alembic upgrade head
+echo "==> Migrations complete."
+
+echo "==> Starting application …"
+exec uvicorn app.main:app --host 0.0.0.0 --port "${PORT:-8080}"