chore: post-deployment cleanup and docs update

- Make AI routing rules DB-driven (read from system_configs, removed from config.py)
- Add translation quota tracking to LLM translation (OpenAIProvider)
- Add Alibaba MT ECS RAM role support (STS token, no AccessKey needed)
- Fix admin sidebar link for AI模型配置 page
- Fix Quota.vue API path (quotas → translation-quotas)
- Fix login auto-redirect to dashboard
- Add provider dropdown selects to AI routing config UI
- Clean up stale ai_provider_* system_configs records
- Remove OpencodeGo, Spark providers (code + DB)
- Update deploy config: nginx port 8000, systemd cwd

backend/alembic/versions/add_payment_transactions_table.py

@@ -9,7 +9,7 @@ import sqlalchemy as sa
 from sqlalchemy.dialects.postgresql import UUID
 
 revision = "add_payment_transactions"
-down_revision = "add_ai_providers_table"
+down_revision = "add_ai_providers"
 branch_labels = None
 depends_on = None