feat(usage): add per-task token usage tracking

Extract token usage from Claude Code's stream-json output in real-time
during task execution, replacing the inaccurate global JSONL log scanner.

- New `task_usage` table: tracks (task_id, provider, model) level usage
- Agent SDK: parse `message.usage` from assistant messages, accumulate
  per-model and return in Result
- Daemon: convert agent usage to entries, send with CompleteTask
- Server: store usage on task completion, expose workspace-level
  aggregation APIs (GET /api/usage/daily, GET /api/usage/summary)

server/pkg/db/generated/models.go

@@ -281,6 +281,18 @@ type TaskMessage struct {
 	CreatedAt pgtype.Timestamptz `json:"created_at"`
 }
 
+type TaskUsage struct {
+	ID               pgtype.UUID        `json:"id"`
+	TaskID           pgtype.UUID        `json:"task_id"`
+	Provider         string             `json:"provider"`
+	Model            string             `json:"model"`
+	InputTokens      int64              `json:"input_tokens"`
+	OutputTokens     int64              `json:"output_tokens"`
+	CacheReadTokens  int64              `json:"cache_read_tokens"`
+	CacheWriteTokens int64              `json:"cache_write_tokens"`
+	CreatedAt        pgtype.Timestamptz `json:"created_at"`
+}
+
 type User struct {
 	ID        pgtype.UUID        `json:"id"`
 	Name      string             `json:"name"`