Refactor config and add comprehensive tests

- Updated config.py to support environment-based configuration
- Added test_config.py with full test coverage
- Updated Backend-Development.md with configuration documentation
- Removed outdated tasks from Tasks.md

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/config.py

@@ -36,9 +36,12 @@ class Settings(BaseSettings):
     )
     session_secret: str = Field(
         ...,
+        min_length=32,
         description=(
             "Secret key used when generating session tokens. "
-            "Must be unique and never committed to source control."
+            "Must be at least 32 characters. "
+            "Must be unique and never committed to source control. "
+            "Generate one with: python -c \"import secrets; print(secrets.token_hex(32))\""
         ),
     )
     session_duration_minutes: int = Field(