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/migrations/032_task_usage.up.sql

@@ -0,0 +1,14 @@
+CREATE TABLE task_usage (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    task_id UUID NOT NULL REFERENCES agent_task_queue(id) ON DELETE CASCADE,
+    provider TEXT NOT NULL DEFAULT '',
+    model TEXT NOT NULL,
+    input_tokens BIGINT NOT NULL DEFAULT 0,
+    output_tokens BIGINT NOT NULL DEFAULT 0,
+    cache_read_tokens BIGINT NOT NULL DEFAULT 0,
+    cache_write_tokens BIGINT NOT NULL DEFAULT 0,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    UNIQUE (task_id, provider, model)
+);
+
+CREATE INDEX idx_task_usage_task_id ON task_usage(task_id);