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/queries/task_usage.sql

@@ -0,0 +1,47 @@
+-- name: UpsertTaskUsage :exec
+INSERT INTO task_usage (task_id, provider, model, input_tokens, output_tokens, cache_read_tokens, cache_write_tokens)
+VALUES ($1, $2, $3, $4, $5, $6, $7)
+ON CONFLICT (task_id, provider, model)
+DO UPDATE SET
+    input_tokens = EXCLUDED.input_tokens,
+    output_tokens = EXCLUDED.output_tokens,
+    cache_read_tokens = EXCLUDED.cache_read_tokens,
+    cache_write_tokens = EXCLUDED.cache_write_tokens;
+
+-- name: GetTaskUsage :many
+SELECT * FROM task_usage
+WHERE task_id = $1
+ORDER BY model;
+
+-- name: GetWorkspaceUsageByDay :many
+SELECT
+    DATE(atq.created_at) AS date,
+    tu.model,
+    SUM(tu.input_tokens)::bigint AS total_input_tokens,
+    SUM(tu.output_tokens)::bigint AS total_output_tokens,
+    SUM(tu.cache_read_tokens)::bigint AS total_cache_read_tokens,
+    SUM(tu.cache_write_tokens)::bigint AS total_cache_write_tokens,
+    COUNT(DISTINCT tu.task_id)::int AS task_count
+FROM task_usage tu
+JOIN agent_task_queue atq ON atq.id = tu.task_id
+JOIN agent a ON a.id = atq.agent_id
+WHERE a.workspace_id = $1
+  AND atq.created_at >= @since::timestamptz
+GROUP BY DATE(atq.created_at), tu.model
+ORDER BY DATE(atq.created_at) DESC, tu.model;
+
+-- name: GetWorkspaceUsageSummary :many
+SELECT
+    tu.model,
+    SUM(tu.input_tokens)::bigint AS total_input_tokens,
+    SUM(tu.output_tokens)::bigint AS total_output_tokens,
+    SUM(tu.cache_read_tokens)::bigint AS total_cache_read_tokens,
+    SUM(tu.cache_write_tokens)::bigint AS total_cache_write_tokens,
+    COUNT(DISTINCT tu.task_id)::int AS task_count
+FROM task_usage tu
+JOIN agent_task_queue atq ON atq.id = tu.task_id
+JOIN agent a ON a.id = atq.agent_id
+WHERE a.workspace_id = $1
+  AND atq.created_at >= @since::timestamptz
+GROUP BY tu.model
+ORDER BY (SUM(tu.input_tokens) + SUM(tu.output_tokens)) DESC;