feat: add translation billing stats and remove Export/Settings pages

- Add TranslationLog model to track translation API usage per task
- Integrate Dify API actual price (total_price) into translation stats
- Display translation statistics in admin dashboard with per-task costs
- Remove unused Export and Settings pages to simplify frontend
- Add GET /api/v2/admin/translation-stats endpoint

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/alembic/versions/g2b3c4d5e6f7_add_translation_logs_table.py

@@ -0,0 +1,58 @@
+"""add_translation_logs_table
+
+Revision ID: g2b3c4d5e6f7
+Revises: f1a2b3c4d5e6
+Create Date: 2025-12-12 10:00:00.000000
+
+Add translation_logs table to track translation API usage and costs.
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = 'g2b3c4d5e6f7'
+down_revision: Union[str, None] = 'f1a2b3c4d5e6'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Create tool_ocr_translation_logs table."""
+    op.create_table(
+        'tool_ocr_translation_logs',
+        sa.Column('id', sa.Integer(), autoincrement=True, nullable=False),
+        sa.Column('user_id', sa.Integer(), nullable=False, comment='Foreign key to users table'),
+        sa.Column('task_id', sa.String(length=255), nullable=False, comment='Task UUID that was translated'),
+        sa.Column('target_lang', sa.String(length=10), nullable=False, comment='Target language code'),
+        sa.Column('source_lang', sa.String(length=10), nullable=True, comment='Source language code'),
+        sa.Column('input_tokens', sa.Integer(), nullable=False, server_default='0', comment='Number of input tokens used'),
+        sa.Column('output_tokens', sa.Integer(), nullable=False, server_default='0', comment='Number of output tokens generated'),
+        sa.Column('total_tokens', sa.Integer(), nullable=False, server_default='0', comment='Total tokens (input + output)'),
+        sa.Column('total_elements', sa.Integer(), nullable=False, server_default='0', comment='Total elements in document'),
+        sa.Column('translated_elements', sa.Integer(), nullable=False, server_default='0', comment='Number of elements translated'),
+        sa.Column('total_characters', sa.Integer(), nullable=False, server_default='0', comment='Total characters translated'),
+        sa.Column('estimated_cost', sa.Float(), nullable=False, server_default='0.0', comment='Estimated cost in USD'),
+        sa.Column('processing_time_seconds', sa.Float(), nullable=False, server_default='0.0', comment='Translation processing time'),
+        sa.Column('provider', sa.String(length=50), nullable=False, server_default='dify', comment='Translation provider'),
+        sa.Column('created_at', sa.DateTime(), nullable=False, server_default=sa.text('CURRENT_TIMESTAMP')),
+        sa.ForeignKeyConstraint(['user_id'], ['tool_ocr_users.id'], ondelete='CASCADE'),
+        sa.PrimaryKeyConstraint('id')
+    )
+    op.create_index(op.f('ix_tool_ocr_translation_logs_id'), 'tool_ocr_translation_logs', ['id'], unique=False)
+    op.create_index(op.f('ix_tool_ocr_translation_logs_user_id'), 'tool_ocr_translation_logs', ['user_id'], unique=False)
+    op.create_index(op.f('ix_tool_ocr_translation_logs_task_id'), 'tool_ocr_translation_logs', ['task_id'], unique=False)
+    op.create_index(op.f('ix_tool_ocr_translation_logs_target_lang'), 'tool_ocr_translation_logs', ['target_lang'], unique=False)
+    op.create_index(op.f('ix_tool_ocr_translation_logs_created_at'), 'tool_ocr_translation_logs', ['created_at'], unique=False)
+
+
+def downgrade() -> None:
+    """Drop tool_ocr_translation_logs table."""
+    op.drop_index(op.f('ix_tool_ocr_translation_logs_created_at'), table_name='tool_ocr_translation_logs')
+    op.drop_index(op.f('ix_tool_ocr_translation_logs_target_lang'), table_name='tool_ocr_translation_logs')
+    op.drop_index(op.f('ix_tool_ocr_translation_logs_task_id'), table_name='tool_ocr_translation_logs')
+    op.drop_index(op.f('ix_tool_ocr_translation_logs_user_id'), table_name='tool_ocr_translation_logs')
+    op.drop_index(op.f('ix_tool_ocr_translation_logs_id'), table_name='tool_ocr_translation_logs')
+    op.drop_table('tool_ocr_translation_logs')