chore(backend): add backend-specific .gitignore

Adds a dedicated .gitignore for the FastAPI/Python backend covering
virtualenvs, build artefacts, test caches, type-checker output,
local SQLite databases, and secrets.

backend/.gitignore

@@ -0,0 +1,49 @@
+# ─────────────────────────────────────────────
+#  backend/.gitignore  (Python / FastAPI)
+# ─────────────────────────────────────────────
+
+# Byte-compiled / optimised source files
+__pycache__/
+*.py[cod]
+*.pyo
+*.pyd
+
+# Virtual environment (local override)
+.venv/
+venv/
+env/
+
+# Distribution / packaging
+dist/
+build/
+*.egg-info/
+
+# Testing
+.coverage
+.coverage.*
+htmlcov/
+.pytest_cache/
+.tox/
+
+# Type checkers & linters
+.mypy_cache/
+.ruff_cache/
+.pytype/
+
+# Local database files
+*.sqlite3
+*.db
+*.db-shm
+*.db-wal
+
+# Alembic generated junk
+alembic/versions/__pycache__/
+
+# Secrets
+.env
+.env.*
+!.env.example
+secrets.json
+
+# Logs
+*.log