feat: multi-issuer OIDC support with config consolidation

- Support multiple OAuth providers (core-api, tatlock-ui, tatlock)
- Changed oidc_issuer (string) to oidc_issuers (list)
- Per-issuer JWKS caching
- Validates token issuer against allowed list
- Consolidated config files (removed deprecated src/config.py, src/security.py)
- Updated imports to use src/shared/config and src/shared/security

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/test_config.py

@@ -1,6 +1,6 @@
 """Tests for config module."""
 import pytest
-from src.config import (
+from src.shared.config import (
     __version__,
     Settings,
     get_settings,