feat: 添加 Dashboard 供应商成本统计功能

- 新增 stats_daily_provider 表存储每日供应商统计数据
- 实现供应商维度的数据聚合服务
- Dashboard API 返回 provider_summary 供应商汇总数据
- 前端新增 DoughnutChart 环形图组件
- Dashboard 新增供应商成本分布可视化卡片
- 移除重复的请求次数/费用趋势折线图

Closes #110

Co-authored-by: RWDai <27391645+RWDai@users.noreply.github.com>

alembic/versions/20260119_0519_c868729753ad_add_stats_daily_provider_table.py

@@ -0,0 +1,65 @@
+"""add_stats_daily_provider_table
+
+Revision ID: c868729753ad
+Revises: 33e347f97c0c
+Create Date: 2026-01-19 05:19:49.634662+00:00
+
+"""
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy import inspect
+
+
+# revision identifiers, used by Alembic.
+revision = 'c868729753ad'
+down_revision = '33e347f97c0c'
+branch_labels = None
+depends_on = None
+
+
+def table_exists(table_name: str) -> bool:
+    """检查表是否存在"""
+    bind = op.get_bind()
+    inspector = inspect(bind)
+    return table_name in inspector.get_table_names()
+
+
+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:
+    """应用迁移：升级到新版本"""
+    if not table_exists('stats_daily_provider'):
+        op.create_table(
+            'stats_daily_provider',
+            sa.Column('id', sa.String(length=36), nullable=False),
+            sa.Column('date', sa.DateTime(timezone=True), nullable=False),
+            sa.Column('provider_name', sa.String(length=100), nullable=False),
+            sa.Column('total_requests', sa.Integer(), nullable=False),
+            sa.Column('input_tokens', sa.BigInteger(), nullable=False),
+            sa.Column('output_tokens', sa.BigInteger(), nullable=False),
+            sa.Column('cache_creation_tokens', sa.BigInteger(), nullable=False),
+            sa.Column('cache_read_tokens', sa.BigInteger(), nullable=False),
+            sa.Column('total_cost', sa.Float(), nullable=False),
+            sa.Column('created_at', sa.DateTime(timezone=True), nullable=False),
+            sa.Column('updated_at', sa.DateTime(timezone=True), nullable=False),
+            sa.PrimaryKeyConstraint('id'),
+            sa.UniqueConstraint('date', 'provider_name', name='uq_stats_daily_provider')
+        )
+        op.create_index('idx_stats_daily_provider_date', 'stats_daily_provider', ['date'], unique=False)
+        op.create_index('idx_stats_daily_provider_date_provider', 'stats_daily_provider', ['date', 'provider_name'], unique=False)
+
+
+def downgrade() -> None:
+    """回滚迁移：降级到旧版本"""
+    if table_exists('stats_daily_provider'):
+        if index_exists('stats_daily_provider', 'idx_stats_daily_provider_date_provider'):
+            op.drop_index('idx_stats_daily_provider_date_provider', table_name='stats_daily_provider')
+        if index_exists('stats_daily_provider', 'idx_stats_daily_provider_date'):
+            op.drop_index('idx_stats_daily_provider_date', table_name='stats_daily_provider')
+        op.drop_table('stats_daily_provider')