feat: add strict mypy type-checking for app/utils/ module

- Add [[tool.mypy.overrides]] section for app/utils/** with disallow_untyped_defs=true
- Add type annotations to all functions in app/utils/ (12 files)
- Fix type annotations in app/config.py and app/database.py (imported by utils)
- All 85 source files now pass mypy type checking

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/config.py

@@ -269,7 +269,7 @@ class Settings(BaseSettings):
 
     @field_validator("notification_urls", mode="before")
     @classmethod
-    def parse_notification_urls(cls, v):
+    def parse_notification_urls(cls, v: str | list[str]) -> list[str]:
         """Parse notification URLs from string or list"""
         if isinstance(v, str):
             if "," in v:
@@ -281,7 +281,7 @@ class Settings(BaseSettings):
 
     @field_validator("session_secret")
     @classmethod
-    def validate_session_secret(cls, v, info):
+    def validate_session_secret(cls, v: str | None, info: object) -> str | None:
         """Validate that session_secret is set and has sufficient length when auth is enabled"""
         if info.data.get("auth_enabled") and not v:
             raise ValueError("SESSION_SECRET must be set when AUTH_ENABLED=True")