feat(M31): agent work routing — scope jobs to assigned agents

Deployment jobs now set agent_id from target→agent relationship at
creation time. GetPendingWork() uses ListPendingByAgentID() with a
3-way UNION query (direct match, legacy NULL fallback via target JOIN,
AwaitingCSR via cert→target→agent chain) so each agent only receives
its own jobs.

- Added AgentID *string to Job domain struct
- Added agent_id to all job SQL queries (5 SELECTs, INSERT, UPDATE, scanJob)
- New ListPendingByAgentID() repository method
- Rewrote GetPendingWork() from ~25 lines to single scoped query
- 4 new Go tests (3 agent routing + 1 deployment agent_id)
- Frontend: agent_id/target_id on Job type

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/src/api/types.ts

@@ -70,6 +70,8 @@ export interface Job {
   id: string;
   certificate_id: string;
   type: string;
+  target_id?: string;
+  agent_id?: string;
   status: string;
   attempts: number;
   max_attempts: number;