ci(security): add Semgrep XXE rule and CI scan job

Add .semgrep/security.yml with rules for DocumentBuilderFactory,
SAXParserFactory, and XMLInputFactory without XXE hardening (CWE-611).
Add semgrep-scan CI job — runs in parallel with backend-unit-tests,
local rules only, --error flag fails the build on any match.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -276,6 +276,26 @@ jobs:
           echo "$dump" | grep -qE "\['add', 'familienarchiv-auth', 'polling'\]" \
             || { echo "FAIL: familienarchiv-auth jail did not resolve to 'polling' backend"; exit 1; }
 
+  # ─── Semgrep Security Scan ───────────────────────────────────────────────────
+  # Catches XXE-unprotected XML parser factories and similar patterns defined in
+  # .semgrep/security.yml. Runs in parallel with backend-unit-tests for fast feedback.
+  # Uses local rules only (no SEMGREP_APP_TOKEN / OIDC — act_runner does not support it).
+  semgrep-scan:
+    name: Semgrep Security Scan
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: '3.11'
+
+      - name: Install Semgrep
+        run: pip install semgrep
+
+      - name: Run security rules
+        run: semgrep --config .semgrep/security.yml --error --metrics=off backend/src/
+
   # ─── Compose Bucket-Bootstrap Idempotency ─────────────────────────────────────
   # docker-compose.prod.yml's create-buckets service runs on every
   # `docker compose up` (one-shot, no restart). Must be idempotent — a

.semgrep/security.yml

@@ -0,0 +1,49 @@
+# Semgrep security rules for Familienarchiv backend.
+# These rules catch the absence of XXE protection on XML parser factories.
+# CWE-611: Improper Restriction of XML External Entity Reference.
+# Run: semgrep --config .semgrep/security.yml --error backend/src/
+
+rules:
+
+  # DocumentBuilderFactory without XXE hardening.
+  # All call sites must call setFeature("…disallow-doctype-decl", true) before use.
+  - id: dbf-xxe-default
+    patterns:
+      - pattern: $X = DocumentBuilderFactory.newInstance();
+      - pattern-not-inside: |
+          ...
+          $X.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);
+          ...
+    message: >
+      DocumentBuilderFactory without XXE protection (CWE-611).
+      Call XxeSafeXmlParser.hardenedFactory() instead of DocumentBuilderFactory.newInstance().
+    languages: [java]
+    severity: WARNING
+
+  # SAXParserFactory without XXE hardening.
+  - id: sax-xxe-default
+    patterns:
+      - pattern: $X = SAXParserFactory.newInstance();
+      - pattern-not-inside: |
+          ...
+          $X.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);
+          ...
+    message: >
+      SAXParserFactory without XXE protection (CWE-611).
+      Apply disallow-doctype-decl and disable external entity features before use.
+    languages: [java]
+    severity: WARNING
+
+  # XMLInputFactory without XXE hardening (StAX parser).
+  - id: stax-xxe-default
+    patterns:
+      - pattern: $X = XMLInputFactory.newInstance();
+      - pattern-not-inside: |
+          ...
+          $X.setProperty(XMLInputFactory.IS_SUPPORTING_EXTERNAL_ENTITIES, false);
+          ...
+    message: >
+      XMLInputFactory without XXE protection (CWE-611).
+      Set IS_SUPPORTING_EXTERNAL_ENTITIES to false and SUPPORT_DTD to false before use.
+    languages: [java]
+    severity: WARNING