feat: granular per-mode alert filters (v0.9.3)

Replace coarse milestone/referral suppression with 8 named action
categories (vote, presidential, committee_report, calendar, procedural,
referral, new_document, new_amendment), each independently togglable
per follow mode (Follow / Pocket Veto / Pocket Boost).

- notification_utils: categorize_action() replaces is_milestone_action /
  is_referral_action; _build_payload stores action_category in payload
- congress_poller: use categorize_action() in _update_bill_if_changed
- notification_dispatcher: _should_dispatch() checks per-mode filter dict
  from notification_prefs; follow mode looked up before filter check
- schemas + api: alert_filters (nested dict) wired through settings
  GET/PUT endpoints; no DB migration required
- frontend: tabbed Alert Filters section (Follow / Pocket Veto /
  Pocket Boost), each with independent 8-toggle filter set, milestone
  parent checkbox (indeterminate-aware), Load defaults button, and
  per-tab Save button

Authored-By: Jack Levy

backend/app/schemas/schemas.py

@@ -42,6 +42,7 @@ class NotificationSettingsResponse(BaseModel):
     quiet_hours_start: Optional[int] = None
     quiet_hours_end: Optional[int] = None
     timezone: Optional[str] = None  # IANA name, e.g. "America/New_York"
+    alert_filters: Optional[dict] = None
 
     model_config = {"from_attributes": True}
 
@@ -59,6 +60,7 @@ class NotificationSettingsUpdate(BaseModel):
     quiet_hours_start: Optional[int] = None
     quiet_hours_end: Optional[int] = None
     timezone: Optional[str] = None  # IANA name sent by the browser on save
+    alert_filters: Optional[dict] = None
 
 
 class NotificationEventSchema(BaseModel):