Fix all 33 ruff linting errors causing CI failures

- Remove unused imports (F401) across 17 files
- Fix f-strings without placeholders (F541) in 3 files
- Add noqa: E712 to SQLAlchemy == True comparisons (valid ORM pattern)
- Preserve alembic side-effect import with noqa: F401

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>
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backend/app/core/config.py

@@ -5,7 +5,7 @@ Supports environment variables and .env files.
 
 from typing import Optional, List
 from pydantic_settings import BaseSettings, SettingsConfigDict
-from pydantic import PostgresDsn, field_validator, ValidationInfo
+from pydantic import field_validator
 
 
 class Settings(BaseSettings):