Implement /clean endpoint for index cleanup

[clafollett]

Description

Implement the /clean endpoint to remove stale and outdated index entries.

Current State

• No implementation
• Defined in OpenAPI spec

Acceptance Criteria

• Implement POST /clean route
• Remove entries for deleted files (file no longer exists)
• Remove entries older than specified duration
• Return cleanup statistics (removed chunks, freed space)
• Add unit tests with mocks
• Add integration tests
• Update OpenAPI documentation

API Request/Response

// Request
{
  "older_than": "7d",        // Duration string
  "missing_files": true        // Remove deleted files
}

// Response
{
  "removed_chunks": 1234,
  "freed_space_mb": 45.6
}

Related

• See api/codetriever-openapi.yaml for full spec