fix(lint): resolve all ruff lint errors

Add ruff config to exclude alembic E402, SQLAlchemy F821, and pre-existing
E501 line-length issues. Auto-fix 69 unused imports and 2 f-strings without
placeholders. Manually fix 8 unused variables. Apply ruff format to 127 files.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/schemas/certificate.py

@@ -10,6 +10,7 @@ from pydantic import BaseModel, ConfigDict
 # Request schemas
 # ---------------------------------------------------------------------------
 
+
 class CACreateRequest(BaseModel):
     """Request to generate a new root CA for the tenant."""
 
@@ -34,6 +35,7 @@ class BulkCertDeployRequest(BaseModel):
 # Response schemas
 # ---------------------------------------------------------------------------
 
+
 class CAResponse(BaseModel):
     """Public details of a tenant's Certificate Authority (no private key)."""