cleanup: remove redundant internal push notification packages

Remove internal/pushprocessor and pushnotif packages that contained duplicate
and unresolved types. All push notification functionality is now consolidated
in the root package with direct type resolution.

Removed Packages:
- internal/pushprocessor/ - contained duplicate Registry, Processor, VoidProcessor
- pushnotif/ - contained interface wrappers that are no longer needed

Benefits:
- Single source of truth for all push notification logic
- No duplicate implementations or unresolved type references
- Cleaner codebase with all functionality in root package
- Eliminated confusion between internal and public interfaces
- Simplified architecture with direct type usage

All functionality remains intact and tests pass. The root package now contains
the complete, self-contained push notification implementation with concrete
types and no external dependencies.

Author: ndyakov