CI: run backoffice test suite on push/PR, document local test setup

Adds .gitea/workflows/test-backoffice.yml (paths-filtered to
backoffice/app/** and backoffice/db/**, self-hosted homelab runner,
plain git clone matching deploy-backoffice.yml's conventions) so a
regression no longer needs a human to notice it.

The psycopg[binary]==3.2.1 pin turned out to be correct, not stale: it
only ships wheels through cp312, so it resolves cleanly under the
project's python:3.12-slim target but not under newer interpreters.
Documented in requirements.txt and docs/agents/testing.md, along with
the fact that the suite spins up its own throwaway postgres:16-alpine
container (tests/conftest.py) with no manual DB setup required.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backoffice/app/requirements.txt

@@ -1,4 +1,10 @@
 Flask==3.0.3
+# psycopg-binary 3.2.1 only publishes prebuilt wheels up to cp312 (see
+# https://pypi.org/project/psycopg-binary/3.2.1/#files) — installing this
+# under Python 3.13+ falls back to a source build (needs libpq headers) or
+# just fails to resolve. Match the python:3.12-slim runtime everywhere
+# (venv, CI, prod image) and this pin installs from wheel with no build
+# toolchain needed. See docs/agents/testing.md.
 psycopg[binary]==3.2.1
 waitress==3.0.0
 PyJWT[crypto]==2.9.0