Add auto-pause TTL and ping endpoint for sandbox inactivity management

Replace the existing auto-destroy TTL behavior with auto-pause: when a
sandbox exceeds its timeout_sec of inactivity, the TTL reaper now pauses
it (snapshot + teardown) instead of destroying it, preserving the ability
to resume later.

Key changes:
- TTL reaper calls Pause instead of Destroy, with fallback to Destroy if
  pause fails (e.g. Firecracker process already gone)
- New PingSandbox RPC resets the in-memory LastActiveAt timer
- New POST /v1/sandboxes/{id}/ping REST endpoint resets both agent memory
  and DB last_active_at
- ListSandboxes RPC now includes auto_paused_sandbox_ids so the reconciler
  can distinguish auto-paused sandboxes from crashed ones in a single call
- Reconciler polls every 5s (was 30s) and marks auto-paused as "paused"
  vs orphaned as "stopped"
- Resume RPC accepts timeout_sec from DB so TTL survives pause/resume cycles
- Reaper checks every 2s (was 10s) and uses a detached context to avoid
  incomplete pauses on app shutdown
- Default timeout_sec changed from 300 to 0 (no auto-pause unless requested)

db/migrations/20260310094104_initial.sql

@@ -8,7 +8,7 @@ CREATE TABLE sandboxes (
     status          TEXT NOT NULL DEFAULT 'pending',
     vcpus           INTEGER NOT NULL DEFAULT 1,
     memory_mb       INTEGER NOT NULL DEFAULT 512,
-    timeout_sec     INTEGER NOT NULL DEFAULT 300,
+    timeout_sec     INTEGER NOT NULL DEFAULT 0,
     guest_ip        TEXT NOT NULL DEFAULT '',
     host_ip         TEXT NOT NULL DEFAULT '',
     created_at      TIMESTAMPTZ NOT NULL DEFAULT NOW(),