Real Hikvision/Dahua camera driver; gate Backend-push-IP on capability

Replace the camera stub with HttpCamera: Hikvision ISAPI and Dahua CGI
snapshots over client-side HTTP Digest (new drivers/http-digest.ts).
healthCheck() now pulls a real frame instead of returning ready/stub.
Snapshot carries bytes (driver fetches); storage/imageRef is the caller's
job, keeping the adapter free of storage deps.

Fix the cosmetic Backend-push-IP field: add pushesToBackend to DeviceDriver
(only Dingtian sets it), expose as pushCapable in the catalog, and gate the
wizard's backend-IP fetch + field on it so pull-only devices hide it.

Verified on hardware (Hikvision 10.0.10.121): healthCheck ready,
captureSnapshot returns a valid JPEG.

apps/web/src/api.ts

@@ -96,6 +96,8 @@ export type DeviceCategory = "access" | "reader" | "camera" | "printer";
 export type Catalog = Record<DeviceCategory, CatalogEntry[]> & {
   /** Driver ids that support LAN discovery. */
   discoverable: string[];
+  /** Driver ids that push to the backend (need a backend IP at assign time). */
+  pushCapable: string[];
 };
 
 export function fetchCatalog(): Promise<Catalog> {