Add mass unban: DELETE /api/bans/all clears all active bans

- Send fail2ban's `unban --all` command via new `unban_all_ips()` service
  function; returns the count of unbanned IPs
- Add `UnbanAllResponse` Pydantic model (message + count)
- Add `DELETE /api/bans/all` router endpoint; handles 502 on socket error
- Frontend: `bansAll` endpoint constant, `unbanAllBans()` API call,
  `UnbanAllResponse` type, `unbanAll` action in `useActiveBans` hook
- JailsPage: "Clear All Bans" button (visible when bans > 0) with a
  Fluent UI confirmation Dialog before executing the operation
- 7 new tests (3 service, 4 router); 440 total pass, 82% coverage

backend/app/models/ban.py

@@ -133,6 +133,15 @@ class ActiveBanListResponse(BaseModel):
     total: int = Field(..., ge=0)
 
 
+class UnbanAllResponse(BaseModel):
+    """Response for ``DELETE /api/bans/all``."""
+
+    model_config = ConfigDict(strict=True)
+
+    message: str = Field(..., description="Human-readable summary of the operation.")
+    count: int = Field(..., ge=0, description="Number of IPs that were unbanned.")
+
+
 # ---------------------------------------------------------------------------
 # Dashboard ban-list view models
 # ---------------------------------------------------------------------------