Add AI provider degradation registry and alerting (server-26#14)

Three AI dependency failures in one night (retired Gemini model IDs,
depleted Gemini balance, unpayable OpenAI account) each surfaced only
as a single ERROR log line that nobody was watching. Add
app/internal/ai_health.py, a shared in-memory registry that
transcription.py and llm_correlator.py report into on every call
(success and failure), distinguishing permanent conditions (dead
model, dead billing) which alert immediately from transient ones
(rate limits, network blips) which only alert after they persist.
Alerts POST once per degradation episode and once on recovery to an
optional Discord webhook (AI_ALERT_WEBHOOK_URL), reusing alerter.py's
httpx pattern. State is exposed unauthenticated at GET /health/ai
alongside the existing /health.

Closes logan/server-26#14

drb-c2-core/app/config.py

@@ -101,6 +101,12 @@ class Settings(BaseSettings):
     # Defaults to "*" for local development only.
     cors_origins: list[str] = ["*"]
 
+    # Discord webhook URL that app/internal/ai_health.py posts to when an AI
+    # tier (transcription/correlation) transitions into or out of degraded
+    # state. Empty disables the POST entirely — not every self-hosted
+    # deployment will set this up, and skipping it must be silent.
+    ai_alert_webhook_url: str = ""
+
     class Config:
         env_file = ".env"