Implement host registration, JWT refresh tokens, and multi-host scheduling

Replaces the hardcoded CP_HOST_AGENT_ADDR single-agent setup with a
DB-driven registration system supporting multiple host agents (BYOC).

Key changes:
- Host agents register via one-time token, receive a 7-day JWT + 60-day
  refresh token; heartbeat loop auto-refreshes on 401/403 and pauses all
  sandboxes if refresh fails
- HostClientPool: lazy Connect RPC client cache keyed by host ID, replacing
  the single static agent client throughout the API and service layers
- RoundRobinScheduler: picks an online host for each new sandbox via
  ListActiveHosts; extensible for future scheduling strategies
- HostMonitor (replaces Reconciler): passive heartbeat staleness check marks
  hosts unreachable and sandboxes missing after 90s; active reconciliation
  per online host restores missing-but-alive sandboxes and stops orphans
- Graceful host delete: returns 409 with affected sandbox list without
  ?force=true; force-delete destroys sandboxes then evicts pool client
- Snapshot delete broadcasts to all online hosts (templates have no host_id)
- sandbox.Manager.PauseAll: pauses all running VMs on CP connectivity loss
- New migration: host_refresh_tokens table with token rotation (issue-then-
  revoke ordering to prevent lockout on mid-rotation crash)
- New sandbox status 'missing' (reversible, unlike 'stopped') and host
  status 'unreachable'; both reflected in OpenAPI spec
- Fix: refresh token auth failure now returns 401 (was 400 via generic
  'invalid' substring match in serviceErrToHTTP)

internal/db/models.go

@@ -36,6 +36,15 @@ type Host struct {
 	MtlsEnabled      bool               `json:"mtls_enabled"`
 }
 
+type HostRefreshToken struct {
+	ID        string             `json:"id"`
+	HostID    string             `json:"host_id"`
+	TokenHash string             `json:"token_hash"`
+	ExpiresAt pgtype.Timestamptz `json:"expires_at"`
+	CreatedAt pgtype.Timestamptz `json:"created_at"`
+	RevokedAt pgtype.Timestamptz `json:"revoked_at"`
+}
+
 type HostTag struct {
 	HostID string `json:"host_id"`
 	Tag    string `json:"tag"`