feat: Add search function and developer guide for implementation

[RK-NerdyBirdy]

This pull request introduces a new search module to the processing package, enabling semantic, keyword, and hybrid search capabilities for user queries. It updates the public API to expose new query-side utilities and provides comprehensive documentation for both indexing and search workflows. The main changes are as follows:

Search Functionality and API Expansion:

• Added a new search.py module that provides query-side search utilities, including functions for building search queries (build_query) and generating query embeddings (query_vector). This module supports vector, keyword, and hybrid (reciprocal rank fusion) search modes, returning SQL and parameters ready for database execution.
• Updated app/processing/__init__.py to publicly export build_query and query_vector, and documented their usage in the module docstring.

Documentation Improvements:

• Expanded the __init__.py docstring to include both indexing (ingestion) and searching (query) workflows, with clear examples and updated module descriptions.
• Added a reference to BACKEND_INTEGRATION.md for a complete setup guide.

Module Structure and Public Surface:

• Updated the modules list in the documentation to include the new search module and clarified the roles of each submodule.
• Updated __all__ in __init__.py to include the new search utilities, ensuring they are accessible from the package's public API.