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)

db/migrations/20260324120214_host_refresh_tokens.sql

@@ -0,0 +1,19 @@
+-- +goose Up
+
+-- Refresh tokens for host agent JWT rotation.
+-- Hosts exchange a refresh token for a new short-lived JWT + new refresh token (rotation).
+-- Refresh tokens expire after 60 days; hosts must re-register with a new one-time token after that.
+CREATE TABLE host_refresh_tokens (
+    id         TEXT PRIMARY KEY,
+    host_id    TEXT NOT NULL REFERENCES hosts(id) ON DELETE CASCADE,
+    token_hash TEXT NOT NULL UNIQUE,   -- SHA-256 hex of the opaque token
+    expires_at TIMESTAMPTZ NOT NULL,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    revoked_at TIMESTAMPTZ             -- NULL = active; set on rotation or host delete
+);
+
+CREATE INDEX idx_host_refresh_tokens_host ON host_refresh_tokens(host_id);
+
+-- +goose Down
+
+DROP TABLE host_refresh_tokens;