Podcast-Feed-Typ im Verwaltungsportal

Passt Verwaltung an die Podcast-Integration im Monitor an (Commit 5127e0a):

Backend (src/routers/sources.py):
- Pydantic-Pattern von GlobalSourceCreate + GlobalSourceUpdate um
  podcast_feed erweitert
- Health-Check Feed-Validierung greift jetzt auch fuer podcast_feed
  (Podcast-Feeds sind technisch RSS/Atom)

Frontend:
- src/static/js/sources.js: TYPE_LABELS um podcast_feed ("Podcast-Feed")
  ergaenzt
- src/static/dashboard.html: Neue <option value=podcast_feed> in Filter-
  und Anlage-Dropdown

Ohne diese Anpassung waere das Anlegen von Podcast-Quellen ueber das
Verwaltungsportal nicht moeglich (422 Unprocessable Entity vom
Pydantic-Validator).

src/routers/sources.py

@@ -34,7 +34,7 @@ class GlobalSourceCreate(BaseModel):
     name: str = Field(min_length=1, max_length=200)
     url: Optional[str] = None
     domain: Optional[str] = None
-    source_type: str = Field(default="rss_feed", pattern="^(rss_feed|web_source|excluded|telegram_channel)$")
+    source_type: str = Field(default="rss_feed", pattern="^(rss_feed|web_source|excluded|telegram_channel|podcast_feed)$")
     category: str = Field(default="sonstige")
     status: str = Field(default="active", pattern="^(active|inactive)$")
     notes: Optional[str] = None
@@ -44,7 +44,7 @@ class GlobalSourceUpdate(BaseModel):
     name: Optional[str] = Field(default=None, max_length=200)
     url: Optional[str] = None
     domain: Optional[str] = None
-    source_type: Optional[str] = Field(default=None, pattern="^(rss_feed|web_source|excluded|telegram_channel)$")
+    source_type: Optional[str] = Field(default=None, pattern="^(rss_feed|web_source|excluded|telegram_channel|podcast_feed)$")
     category: Optional[str] = None
     status: Optional[str] = Field(default=None, pattern="^(active|inactive)$")
     notes: Optional[str] = None
@@ -597,7 +597,7 @@ async def run_health_check_stream(
                                 "message": f"HTTP {resp.status_code} - nicht erreichbar"})
                         else:
                             checks.append({"type": "reachability", "status": "ok", "message": "Erreichbar"})
-                            if source["source_type"] == "rss_feed":
+                            if source["source_type"] in ("rss_feed", "podcast_feed"):
                                 text = resp.text[:20000]
                                 if "<rss" not in text and "<feed" not in text and "<channel" not in text:
                                     checks.append({"type": "feed_validity", "status": "error",