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/api/agent_doc_check_routes.py

@@ -51,6 +51,7 @@ class CheckItem(BaseModel):
     level: int = 1
     parent: str | None = None
     skipped: bool = False
+    hint: str = ""
 
 
 class DocCheckResult(BaseModel):
@@ -247,6 +248,7 @@ def _run_checklist(text: str, doc_type: str, label: str, url: str, word_count: i
                     level=c.get("level", 1),
                     parent=c.get("parent"),
                     skipped=c.get("skipped", False),
+                    hint=c.get("hint", ""),
                 ))
             completeness = f.get("completeness_pct", 0)
             correctness = f.get("correctness_pct", 0)