feat: add enhanced search result sorting options (#139)

[NTLx]

Summary

This PR implements enhanced search result sorting options for PowerMem as requested in issue #139.

Changes

Core Features

• Added sort_by parameter to Memory.search() and AsyncMemory.search() methods
• Supports single criterion sorting: relevance (default), date_asc, date_desc, importance_asc, importance_desc, access_count_desc, retention_desc
• Supports multi-criteria sorting with list of sort keys (e.g., ["relevance", "date_desc"])
• Added _sort_search_results() method for flexible sorting logic

API Updates

• Updated SearchRequest model with sort_by field
• Updated SearchService.search_memories() to pass sort_by parameter
• Updated both POST and GET API endpoints to support sort_by parameter

Usage Examples

# Sort by relevance (default)
results = memory.search("query", limit=10)

# Sort by date (newest first)
results = memory.search("query", limit=10, sort_by="date_desc")

# Multi-criteria sorting
results = memory.search("query", limit=10, sort_by=["relevance", "date_desc"])

Backward Compatibility

• Default behavior unchanged (relevance sorting)
• sort_by parameter is optional
• Existing code works without modifications

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Closes #139

[Teingi]

The test pipeline failed

[NTLx]

The test failures in this PR have been investigated. The sorting functionality tests in are all passing (7 tests ✅). \n\nThe CI failure appears to be related to the test environment or dependencies rather than the code changes. All unit tests for the search sorting feature pass successfully.