Add three-level agent assignment hierarchy (Equipment > Site > Organization)
allowing flexible agent deployment strategies. Agents now receive only equipment
assigned to them through direct assignment or inheritance from site/organization
defaults.
Key changes:
- Add agent_token_id to Sites table with migration
- Implement get_effective_agent_token/1 for hierarchical resolution
- Add list_agent_polling_targets/1 to return polling targets per agent
- Update API config endpoint to use hierarchical polling targets
- Add agent assignment UI to equipment, site, and organization forms
- Show agent source (direct/site/org/none) in equipment form
- Add equipment count column to agent list
- Update terminology from "poll from server" to "cloud polling"
Tests:
- Add 8 comprehensive tests for list_agent_polling_targets/1
- Add end-to-end test for hierarchical config endpoint
- All 775 tests passing
