feat: Telegram alert on regime quadrant change (hysteresis + cooldown)

Fires once when the regime monitor shifts quadrant (regime index x early
warning), so you don't have to watch the tab. Two guards against spam:

- Hysteresis: each axis only flips once the value crosses its divider by a
  margin, so a point parked on a boundary keeps its quadrant instead of
  flip-flopping day to day.
- Cooldown: a genuine change stays quiet for a few days after the last alert.

Seeds the baseline silently on first run; reuses the existing Telegram dispatch
+ AlertLog. New per-trigger toggle in Admin → Alerts (on by default).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/schemas/admin.py

@@ -98,3 +98,4 @@ class AlertConfigUpdate(BaseModel):
     sr_proximity_enabled: bool | None = None
     score_drop_enabled: bool | None = None
     digest_enabled: bool | None = None
+    regime_quadrant_enabled: bool | None = None