feat: [US-001] Add automatic Alembic migrations at backend startup

- Add alembic.ini config for data_entry module
- Add migrations.py helper with:
  - run_migrations(): runs pending migrations at startup (returns True/False)
  - get_current_revision(): returns current schema version for /health
- Integrate migrations in main.py startup_event() BEFORE init_data_entry_db()
- Add data_entry_schema_version to /health endpoint

If migrations fail, backend continues with WARNING (not crash).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.claude/settings.json

@@ -23,6 +23,8 @@
       "Bash(ruff format*)",
       "Bash(mypy*)",
       "Bash(pip install*)",
+      "Bash(cd backend && alembic*)",
+      "Bash(python -m py_compile*)",
       "Bash(git checkout*)",
       "Bash(git status)",
       "Bash(git diff*)",