Slice 3 patch for the generic search platform.

Contents:
- long-text CHUNK representations for generic and TED documents
- representation selection mode for generic search (PRIMARY_ONLY / PRIMARY_AND_CHUNKS / ALL)
- chunk-aware document collapse and matchedRepresentationCount in fused results
- recency-aware scoring boost
- lightweight search metrics endpoint: GET /api/search/metrics

Assumptions:
- apply on top of Slice 2 and the Slice 2 fix patch
- no additional DB migration is required in this slice

Notes:
- Maven compile was not available in the patch generation environment
- this patch intentionally keeps TED and Mail structured search for later slices
