feat: add camera location hierarchy

[matteius]

Summary

• add stable, adjacency-list camera locations with a seeded immutable Unassigned root
• preserve camera assignments through ordinary config changes and expose location UUIDs through camera APIs and backups
• add cycle-safe hierarchy CRUD and UUID-based camera assignment endpoints, initially admin-only pending scoped authorization
• add database and handler coverage for hierarchy moves, conflicts, assignments, migration defaults, metadata, and auth

API

• GET /api/locations
• POST /api/locations
• GET|PUT|DELETE /api/locations/{location_uuid}
• PUT /api/cameras/{camera_uuid}/location

Validation

• test_config
• test_db_streams
• test_db_locations
• test_api_handlers_locations
• test_api_handlers_system
• upgrade smoke test: existing cameras backfilled to the seeded Unassigned location
• git diff --check

Stack

• depends on feat: add stable camera UUIDs #497 (stable camera UUIDs)
• roadmap/PRDs: docs: define fleet control plane PRD roadmap #496

Part of FLEET 01 P1.