style: apply black and isort formatting across entire codebase

- Standardize quote style (single to double quotes)
- Reorder and group imports alphabetically
- Fix line breaks and indentation for consistency
- Apply PEP 8 formatting standards

Also updated Makefile to exclude both venv and .venv from code quality checks.

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

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

models/schema/auth.py

@@ -17,7 +17,9 @@ class UserRegister(BaseModel):
     @classmethod
     def validate_username(cls, v):
         if not re.match(r"^[a-zA-Z0-9_]+$", v):
-            raise ValueError("Username must contain only letters, numbers, or underscores")
+            raise ValueError(
+                "Username must contain only letters, numbers, or underscores"
+            )
         return v.lower().strip()
 
     @field_validator("password")
@@ -31,7 +33,9 @@ class UserRegister(BaseModel):
 class UserLogin(BaseModel):
     email_or_username: str = Field(..., description="Username or email address")
     password: str = Field(..., description="Password")
-    vendor_code: Optional[str] = Field(None, description="Optional vendor code for context")
+    vendor_code: Optional[str] = Field(
+        None, description="Optional vendor code for context"
+    )
 
     @field_validator("email_or_username")
     @classmethod