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/api/agent_helper.go

@@ -0,0 +1,20 @@
+package api
+
+import (
+	"context"
+	"fmt"
+
+	"git.omukk.dev/wrenn/sandbox/internal/db"
+	"git.omukk.dev/wrenn/sandbox/internal/lifecycle"
+	"git.omukk.dev/wrenn/sandbox/proto/hostagent/gen/hostagentv1connect"
+)
+
+// agentForHost looks up the host record and returns a Connect RPC client for it.
+// Returns an error if the host is not found or has no address.
+func agentForHost(ctx context.Context, queries *db.Queries, pool *lifecycle.HostClientPool, hostID string) (hostagentv1connect.HostAgentServiceClient, error) {
+	host, err := queries.GetHost(ctx, hostID)
+	if err != nil {
+		return nil, fmt.Errorf("host not found: %w", err)
+	}
+	return pool.GetForHost(host)
+}