Add a real pytest suite, gating the CI build

64 tests covering: auth/setup and the CSRF gate, the "owner adds their
own data, anyone linked can mute it" permission pattern shared across
calendar-select/tasks-source/whiteboard-source, migration correctness
(fresh install, idempotent re-run, expected columns), battery estimate
outlier rejection, calendar_feed's fetch/merge/partial-failure handling,
webdav_client's fetch/list-directory, the whiteboard force-refresh
throttle bypass and browse endpoint, and render-size invariants across
calendar views/orientations.

No DB/HTTP fixtures need Docker, Node, or a real Immich/CalDAV/WebDAV
server -- a fresh temp SQLite file plus a couple of small local HTTP
servers as test doubles cover it all. Table data is wiped and reseeded
between tests rather than relying on SQLAlchemy's transaction-rollback
isolation pattern, which needs a pysqlite event-listener workaround
app/db.py's engine doesn't have and has no reason to gain just for tests.

Wired into .gitea/workflows/server-docker-build.yml as its own job that
build-and-push now depends on, so a failing suite blocks the image push
rather than just running alongside it for show.

.gitea/workflows/server-docker-build.yml

@@ -8,7 +8,27 @@ on:
       - ".gitea/workflows/server-docker-build.yml"
 
 jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: "3.12"
+
+      - name: Install dependencies
+        working-directory: server
+        run: pip install -r requirements-dev.txt
+
+      - name: Run tests
+        working-directory: server
+        run: pytest
+
   build-and-push:
+    needs: test
     runs-on: ubuntu-latest
     steps:
       - name: Checkout