Phase 2 Health-Check tenant-fähig + Historie

- migrations/2026-05-09d_source_health_history.py NEU: source_health_history-Tabelle
  (Append-only Verlauf der Health-Check-Runs mit run_id und archived_at)
- shared/services/source_health.py:
  - tenant_id IS NULL Filter raus -> auch Tenant-Quellen werden gecheckt
  - Mojibake (Triple-Encoded UTF-8) via ftfy gefixt
  - DELETE FROM source_health_checks: vorher Stand mit run_id (uuid4) in
    source_health_history archivieren -> kein Datenverlust mehr
  - User-Agent + Timeout aus config.HEALTH_CHECK_* statt hardcoded
- routers/sources.py /health/run-stream: gleiche Änderungen wie oben
- config.py: HEALTH_CHECK_USER_AGENT + HEALTH_CHECK_TIMEOUT_S ergänzt

src/config.py

@@ -48,3 +48,10 @@ MAX_FEEDS_PER_DOMAIN = 3
 CLAUDE_MODEL_FAST = "claude-haiku-4-5-20251001"
 CLAUDE_MODEL_MEDIUM = "claude-sonnet-4-6"
 CLAUDE_MODEL_STANDARD = "claude-opus-4-7"
+
+# Health-Check (genutzt von shared/services/source_health.py + routers/sources.py)
+HEALTH_CHECK_USER_AGENT = os.environ.get(
+    "HEALTH_CHECK_USER_AGENT",
+    "Mozilla/5.0 (compatible; AegisSight-HealthCheck/1.0)",
+)
+HEALTH_CHECK_TIMEOUT_S = float(os.environ.get("HEALTH_CHECK_TIMEOUT_S", "15.0"))