Implement hybrid fusion + reranker and staged status UI

[hammertoe]

Summary

Implement robust hybrid retrieval with RRF fusion + Gemini reranker, and replace typing dots with staged, user-friendly progress statuses (SSE-backed with fallback).

Retrieval: Robust Fusion + Reranker

• Split seed retrieval into separate channels (vector, full-text, alias) and keep ranked lists distinct.
• Add RRF fusion helper (reciprocal rank fusion) with configurable .
• Add intent boosts (topic term coverage, recency intent) and generic-term penalties.
• Add Gemini rerank step over top N fused candidates with strict JSON schema; fallback to fused order on failure/timeout.
• Feed reranked seeds into edge + citation retrieval; enforce topical evidence minimum for topical queries.
• Add feature flags in config: , , , , .

Progress Events (Backend)

• Add stage callbacks around chat pipeline in :
  • received
  • retrieving_sources
  • ranking_sources
  • reading_evidence
  • drafting_answer
  • finalizing_response
• Add SSE endpoint in for streaming stage events + final response.
• Keep existing POST endpoint for compatibility.

UI: Staged Status Messages

• Add SSE client in (event types: stage, final, error).
• Replace bouncing dots with staged status card in .
• Friendly stage copy (non-technical):
  • Got it — checking your question.
  • Looking through recent debates and documents.
  • Picking the most relevant receipts from Parliament.
  • Reading the clips and bill excerpts.
  • Drafting your answer.
  • Adding sources and final checks.
• Add fallback staged progression if SSE unavailable.
• Update styles in .

Tests

• Add unit tests for fusion behavior (generic lexical dominance doesn’t suppress topical vector hits).
• Add tests for reranker fallback on malformed JSON/timeouts.
• Add tests for topical evidence fallback response.

Rollout

• Feature-flagged rollout on staging.
• Evaluate: topical source precision, off-topic drift rate, latency impact, user reformulation rate.