HA: unified server-backed on-video widget layer (camera_overlays) folding in PTZ

[badbread]

Audit gap (epic #52 P4 signature deliverable). No camera_overlays table; HA overlays ride camera_ha_links columns (cross-device) but PTZ custom panels persist to a client-local store, so PTZ placements are invisible cross-device (the 'root weirdness' the epic set out to kill). Unify into one server-backed declarative widget layer rendered 3 ways, migrate PTZ off local storage. Large/strategic. Part of the HA management overhaul epic.

[badbread]

Moved out of the HA management overhaul (#445) into its own strategic epic #451 after a plan review: this is a large PTZ-centric architectural refactor, not HA management completeness. When picked up, pin the camera_overlays data model first.

[badbread]

Deferred (backlog). Architectural refactor (unify PTZ + HA into one server-backed camera_overlays widget layer), tracked under epic #451. No user-facing urgency. Trigger: the overlay system needs unifying for another reason.