feat(usage): add Hermes Agent tracking + fix zero-cost bug + perf

Hermes:
- Parse ~/.hermes/state.db sessions (incl. profiles/*/state.db) into
  proxy_request_logs with data_source='hermes_session', WAL-aware
  incremental sync, Hermes-reported cost preferred over model_pricing
  fallback

Zero-cost bug (dashboard showed \$0 totals):
- GPT-5.5 family default pricing (~83% of affected rows used GPT-5.5)
- find_model_pricing_row: ASCII-lowercase normalization so
  "OpenAI/GPT-5.5@HIGH" matches seeded "gpt-5.5"
- Startup cost backfill in async task: scan rows where total_cost <= 0
  but tokens > 0, recompute via model_pricing in a single transaction

Performance:
- Add (app_type, created_at DESC) covering index for dashboard range
  queries
- Add expression index on COALESCE(data_source, 'proxy') so dedup EXISTS
  subqueries use index lookup instead of full scan; drop superseded
  idx_request_logs_dedup_lookup

Refactor:
- row_to_request_log_detail helper (3-way de-dup; fixes cost_multiplier
  \"1\" vs \"1.0\" drift between callers)
- Promote get_sync_state/update_sync_state to shared session_usage
  module (4 copies -> 1)
- run_step helper in lib.rs replaces 9 if-let-Err blocks
- maybe_backfill_log_costs returns bool to skip duplicate total_cost
  parsing in caller

src-tauri/src/services/session_usage.rs

@@ -308,8 +308,10 @@ fn sync_single_file(db: &Database, file_path: &Path) -> Result<(u32, u32), AppEr
     Ok((imported, skipped))
 }
 
-/// 获取文件的同步状态
-fn get_sync_state(db: &Database, file_path: &str) -> Result<(i64, i64), AppError> {
+/// 获取 session_log_sync 表中某条目的同步进度。
+///
+/// Shared by all session_usage_* parsers.
+pub(crate) fn get_sync_state(db: &Database, file_path: &str) -> Result<(i64, i64), AppError> {
     let conn = lock_conn!(db.conn);
     let result = conn.query_row(
         "SELECT last_modified, last_line_offset FROM session_log_sync WHERE file_path = ?1",
@@ -319,8 +321,10 @@ fn get_sync_state(db: &Database, file_path: &str) -> Result<(i64, i64), AppError
     Ok(result.unwrap_or((0, 0)))
 }
 
-/// 更新文件的同步状态
-fn update_sync_state(
+/// 更新 session_log_sync 表中某条目的同步进度。
+///
+/// Shared by all session_usage_* parsers.
+pub(crate) fn update_sync_state(
     db: &Database,
     file_path: &str,
     last_modified: i64,