fix(ci): fetch full git history in ai-credits job for diff-based change detection

Previous job died silently right after checkout with no logs at all
from the 'Check for relevant changes' step onward - shallow clone
(default fetch-depth:1, unlike build.yaml which sets fetch-depth:0)
may have been the cause. Matching build.yaml's checkout config.

Also re-set TEST_DATABASE_URL with a properly percent-encoded password
(the raw '@' in it made the postgresql:// URI ambiguous to parse).

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

.forgejo/workflows/test.yaml

@@ -18,6 +18,8 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
 
       - name: Check for relevant changes
         id: check