feat(sources): externer Reputations-Layer (IFCN + EUvsDisinfo)

Externe Datenquellen (kostenlos, Open Data) ergaenzen die LLM-geschaetzte
Reliability-Achse mit objektiven Signalen:

- IFCN-Signatories (raw.githubusercontent.com/IFCN/verified-signatories):
  Plain-Text-Liste anerkannter Faktencheck-Organisationen.
- EUvsDisinfo (Zenodo CSV): Pro-Kreml-Desinformations-Datenbank.

Schema-Erweiterung:
- ifcn_signatory, eu_disinfo_listed, eu_disinfo_case_count,
  eu_disinfo_last_seen, external_data_synced_at.

Service src/services/external_reputation.py:
- sync_ifcn_signatories(), sync_eu_disinfo(), apply_reputation_overrides(),
  sync_all() mit Domain-Normalisierung (lowercase, ohne www., ohne Schema).

Reliability-Override-Regeln (laufen nach Approve und manuellem Sync):
- ifcn_signatory=1 -> reliability=sehr_hoch
- eu_disinfo_case_count >= 5 -> reliability=sehr_niedrig
- eu_disinfo_case_count >= 1 -> Reliability eine Stufe runter (max niedrig)

API: POST /api/sources/external-reputation/sync (Admin, BackgroundTask).
Filter: ?ifcn_signatory=true, ?eu_disinfo_listed=true.

UI:
- Filter-Dropdown "Externe Reputation" im Quellen-Modal.
- Badges: gruenes "IFCN" und rotes "EU-Desinfo (n)".
- Tooltip macht Reliability-Quelle transparent: "(IFCN-Faktenchecker)",
  "(EU-Desinfo, n Faelle)" oder "(LLM-Schaetzung)".
- "Externe Daten syncen"-Button im Review-Toolbar (Admin-only).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/models.py

@@ -210,6 +210,11 @@ class SourceResponse(BaseModel):
     classified_at: Optional[str] = None
     alignments: list[str] = []
     is_global: bool = False
+    ifcn_signatory: bool = False
+    eu_disinfo_listed: bool = False
+    eu_disinfo_case_count: int = 0
+    eu_disinfo_last_seen: Optional[str] = None
+    external_data_synced_at: Optional[str] = None
 
 
 # Source Discovery