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- Removed outdated tasks from Tasks.md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/tests/test_config.py

@@ -105,3 +105,56 @@ def test_fail2ban_start_command_multiple_arguments() -> None:
         fail2ban_start_command="fail2ban-client -c /etc/fail2ban start",
     )
     assert settings.fail2ban_start_command == "fail2ban-client -c /etc/fail2ban start"
+
+
+def test_session_secret_enforces_minimum_length() -> None:
+    """session_secret must be at least 32 characters."""
+    # Test with a secret shorter than 32 characters
+    with pytest.raises(ValidationError) as exc_info:
+        Settings(
+            database_path="/tmp/test.db",
+            fail2ban_socket="/tmp/fake_fail2ban.sock",
+            fail2ban_config_dir="/tmp/fail2ban",
+            session_secret="short-secret",
+        )
+    error_msg = str(exc_info.value)
+    assert "session_secret" in error_msg
+    assert "32" in error_msg or "at least" in error_msg
+
+
+def test_session_secret_accepts_32_characters() -> None:
+    """session_secret with exactly 32 characters is accepted."""
+    secret_32 = "a" * 32  # Exactly 32 characters
+    settings = Settings(
+        database_path="/tmp/test.db",
+        fail2ban_socket="/tmp/fake_fail2ban.sock",
+        fail2ban_config_dir="/tmp/fail2ban",
+        session_secret=secret_32,
+    )
+    assert settings.session_secret == secret_32
+
+
+def test_session_secret_accepts_longer_than_32() -> None:
+    """session_secret with more than 32 characters is accepted."""
+    secret_64 = "b" * 64  # 64 characters
+    settings = Settings(
+        database_path="/tmp/test.db",
+        fail2ban_socket="/tmp/fake_fail2ban.sock",
+        fail2ban_config_dir="/tmp/fail2ban",
+        session_secret=secret_64,
+    )
+    assert settings.session_secret == secret_64
+
+
+def test_session_secret_error_message_includes_guidance() -> None:
+    """Validation error for short session_secret includes secret generation guidance."""
+    with pytest.raises(ValidationError) as exc_info:
+        Settings(
+            database_path="/tmp/test.db",
+            fail2ban_socket="/tmp/fake_fail2ban.sock",
+            fail2ban_config_dir="/tmp/fail2ban",
+            session_secret="too-short",
+        )
+    error_msg = str(exc_info.value)
+    # Verify the error mentions the constraint
+    assert "session_secret" in error_msg