Fix CodeRabbit code quality issues across repository, schema, auth, and core modules

[Amanbig]

PR Title

Fix CodeRabbit code quality issues across repository, schema, auth, and core modules

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Description

This PR addresses multiple code quality issues identified by CodeRabbit across several modules.

repository.py

• Replaced deprecated datetime.utcnow() with datetime.now(timezone.utc) for Python 3.12+ compatibility
• Fixed _to_dict() fallback to safely handle non-Pydantic models using __dict__ instead of failing dict(model) call

schemas.py

• Added None checks for spec_from_file_location() and spec.loader before use
• Removed deprecated json_encoders from ConfigDict (Pydantic v2 handles datetime serialization natively)
• Fixed create_update_model() to properly set field defaults by adding values to the namespace dictionary
• Added a warning when registering schema short names that shadow existing schemas

auth.py

• Replaced deprecated datetime.utcnow() with datetime.now(timezone.utc)
• Changed require_roles() and require_permissions() from async to sync methods
  (they return async checker functions, not coroutines)
• Fixed get_current_active_user dependency injection that was incorrectly creating new AuthDependencies instances

core.py

• Fixed module name construction to avoid double dots when prefix is empty (e.g. routes..name)
• Added None checks for spec_from_file_location() and spec.loader

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Related Issue

Closes #13
(Remove or update this line if not applicable)

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Type of Change

• Bug fix
• New feature
• Documentation update
• Tests
• Refactor

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How Has This Been Tested?

• Unit tests (existing 20 tests pass)
• Manual testing
• Not tested (explain why)

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Checklist

• My code follows the project style guidelines
• I have added tests (if applicable)
• I have updated documentation (if needed)
• I have linked the related issue