feat(agent): support multiple agents with shared token and self-reported IDs

- Add AgentID field to StatusReport for agent self-identification
- Implement RegisterAgent() method for dynamic agent registration
- Modify agentReportStatusHandler to extract and use agent-reported IDs
- Use wildcard credential to support multiple agents with INTERNAL_SERVICE_TOKEN
- Add agents table schema for future persistence

This enables scalable multi-node deployment without pre-configuring each agent ID.

sql/20260205_agents_table.sql

@@ -0,0 +1,22 @@
+-- Agent registration and health tracking
+-- Migration: 20260205_agents_table.sql
+
+CREATE TABLE IF NOT EXISTS public.agents (
+    id TEXT PRIMARY KEY,                      -- Agent ID (e.g., "hk-xhttp.svc.plus")
+    name TEXT NOT NULL DEFAULT '',            -- Display name
+    groups TEXT[] NOT NULL DEFAULT '{}',      -- Agent groups (e.g., {"internal"})
+    healthy BOOLEAN NOT NULL DEFAULT false,   -- Last reported health status
+    last_heartbeat TIMESTAMPTZ,               -- Last successful heartbeat time
+    clients_count INTEGER NOT NULL DEFAULT 0, -- Number of Xray clients
+    sync_revision TEXT,                       -- Last sync revision
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+CREATE INDEX IF NOT EXISTS idx_agents_last_heartbeat ON public.agents(last_heartbeat);
+CREATE INDEX IF NOT EXISTS idx_agents_healthy ON public.agents(healthy);
+
+COMMENT ON TABLE public.agents IS 'Registered agents with health tracking and automatic cleanup';
+COMMENT ON COLUMN public.agents.id IS 'Self-reported agent ID from StatusReport.agentId';
+COMMENT ON COLUMN public.agents.last_heartbeat IS 'Last successful heartbeat timestamp, used for stale agent cleanup';
+COMMENT ON COLUMN public.agents.groups IS 'Agent groups inherited from authentication credential';