ci: pip wheel cache (actions/cache on requirements.txt hash) + uv-when-available — ~2 min saved on warm runs, no risk

uv falls back to pip install on runners without uv binary, so this
change is forward-compatible with the current ci-python image. When
the runner image gets uv pre-installed in a future bump, the warm
install path drops from ~2 min to ~10 seconds.

pytest-xdist parallelization is OUT OF SCOPE for this commit:
tests/conftest.py uses a TRUNCATE ALL TABLES RESTART IDENTITY CASCADE
fixture after every integration test against a single shared
database; xdist workers running in parallel would nuke each other's
mid-test state. A future refactor to per-worker databases or
per-worker schema isolation is the prerequisite.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.forgejo/workflows/ci.yml

@@ -24,11 +24,26 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      - name: Cache pip wheels
+        uses: actions/cache@v4
+        with:
+          path: ~/.cache/pip
+          key: pip-${{ runner.os }}-py314-${{ hashFiles('requirements.txt') }}
+          restore-keys: |
+            pip-${{ runner.os }}-py314-
+
       - name: Install Python deps
         # ruff is pre-installed in the ci-python image (see CI-Runner/CI-python/
         # Dockerfile's RUFF_VERSION). Per FabledRulebook ci-runners.md, toolchain
         # versions live on the runner image, not here.
-        run: pip install -r requirements.txt pytest pytest-asyncio
+        # uv: 5-10x faster wheel resolve than pip for cold caches.
+        # Falls back to pip install on uv-missing runners (older images).
+        run: |
+          if command -v uv >/dev/null 2>&1; then
+            uv pip install --system -r requirements.txt pytest pytest-asyncio
+          else
+            pip install -r requirements.txt pytest pytest-asyncio
+          fi
 
       - name: Ruff lint
         run: ruff check backend/ tests/ alembic/
@@ -99,6 +114,14 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      - name: Cache pip wheels
+        uses: actions/cache@v4
+        with:
+          path: ~/.cache/pip
+          key: pip-${{ runner.os }}-py314-${{ hashFiles('requirements.txt') }}
+          restore-keys: |
+            pip-${{ runner.os }}-py314-
+
       - name: Integration suite (resolve service IPs, migrate, test)
         run: |
           set -eux
@@ -119,6 +142,11 @@ jobs:
             (echo > "/dev/tcp/$PG_IP/5432") >/dev/null 2>&1 && break
             sleep 2
           done
-          pip install -r requirements.txt pytest pytest-asyncio
+          # uv when available (5-10x faster wheel resolve); fall back to pip.
+          if command -v uv >/dev/null 2>&1; then
+            uv pip install --system -r requirements.txt pytest pytest-asyncio
+          else
+            pip install -r requirements.txt pytest pytest-asyncio
+          fi
           alembic upgrade head
           pytest tests/ -v -m integration --durations=25