feat: Add actionable hints to all 138 compliance checks

Each check now has a "hint" field explaining what is missing and
what the customer should do to fix it. Hints are shown in the
frontend below failed checks in red text.

Examples:
- "Bei Verarbeitung auf Basis von Art. 6(1)(f) muss dokumentiert
  werden, warum Ihr berechtigtes Interesse die Rechte der
  Betroffenen ueberwiegt."
- "Die ladungsfaehige Anschrift fehlt. Erforderlich: Strasse,
  Hausnummer, PLZ und Ort."

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend-compliance/compliance/services/doc_checks/runner.py

@@ -129,6 +129,7 @@ def check_document_completeness(
             "passed": passed, "severity": check.get("severity", "MEDIUM"),
             "matched_text": _extract_context(text_lower, match),
             "level": 1, "parent": None, "skipped": False,
+            "hint": check.get("hint", ""),
         })
 
     # ── Pass 2: L2 checks (only if parent L1 passed) ─────────────────
@@ -165,6 +166,7 @@ def check_document_completeness(
             "passed": passed, "severity": check.get("severity", "MEDIUM"),
             "matched_text": matched_text,
             "level": 2, "parent": parent, "skipped": skipped,
+            "hint": check.get("hint", ""),
         })
 
     # ── Summary ───────────────────────────────────────────────────────