feat: add TTFB timeout detection and improve stream handling

- Add stream first byte timeout (TTFB) detection to trigger failover
  when provider responds too slowly (configurable via STREAM_FIRST_BYTE_TIMEOUT)
- Add rate limit fail-open/fail-close strategy configuration
- Improve exception handling in stream prefetch with proper error classification
- Refactor UsageService with shared _prepare_usage_record method
- Add batch deletion for old usage records to avoid long transaction locks
- Update CLI adapters to use proper User-Agent headers for each CLI client
- Add composite indexes migration for usage table query optimization
- Fix streaming status display in frontend to show TTFB during streaming
- Remove sensitive JWT secret logging in auth service

alembic/versions/20251220_1500_add_usage_composite_indexes.py

@@ -0,0 +1,65 @@
+"""add usage table composite indexes for query optimization
+
+Revision ID: b2c3d4e5f6g7
+Revises: a1b2c3d4e5f6
+Create Date: 2025-12-20 15:00:00.000000+00:00
+
+"""
+from alembic import op
+from sqlalchemy import inspect
+
+# revision identifiers, used by Alembic.
+revision = 'b2c3d4e5f6g7'
+down_revision = 'a1b2c3d4e5f6'
+branch_labels = None
+depends_on = None
+
+
+def index_exists(table_name: str, index_name: str) -> bool:
+    """检查索引是否存在"""
+    bind = op.get_bind()
+    inspector = inspect(bind)
+    indexes = [idx['name'] for idx in inspector.get_indexes(table_name)]
+    return index_name in indexes
+
+
+def upgrade() -> None:
+    """为 usage 表添加复合索引以优化常见查询"""
+    # 1. user_id + created_at 复合索引 (用户用量查询)
+    if not index_exists('usage', 'idx_usage_user_created'):
+        op.create_index(
+            'idx_usage_user_created',
+            'usage',
+            ['user_id', 'created_at'],
+            postgresql_concurrently=True
+        )
+
+    # 2. api_key_id + created_at 复合索引 (API Key 用量查询)
+    if not index_exists('usage', 'idx_usage_apikey_created'):
+        op.create_index(
+            'idx_usage_apikey_created',
+            'usage',
+            ['api_key_id', 'created_at'],
+            postgresql_concurrently=True
+        )
+
+    # 3. provider + model + created_at 复合索引 (模型统计查询)
+    if not index_exists('usage', 'idx_usage_provider_model_created'):
+        op.create_index(
+            'idx_usage_provider_model_created',
+            'usage',
+            ['provider', 'model', 'created_at'],
+            postgresql_concurrently=True
+        )
+
+
+def downgrade() -> None:
+    """删除复合索引"""
+    if index_exists('usage', 'idx_usage_provider_model_created'):
+        op.drop_index('idx_usage_provider_model_created', table_name='usage')
+
+    if index_exists('usage', 'idx_usage_apikey_created'):
+        op.drop_index('idx_usage_apikey_created', table_name='usage')
+
+    if index_exists('usage', 'idx_usage_user_created'):
+        op.drop_index('idx_usage_user_created', table_name='usage')