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>

.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