Narrow fuzzy and regex search candidates

[dutifulbob]

Summary

• move regex and fuzzy mention search onto narrowed candidate scans instead of whole-repo scans
• keep Postgres only on the fast FTS path
• use normalized text fragments to prefilter candidate documents before Go-side scoring

Testing

• go test ./internal/searchindex/... ./internal/httpapi/... ./internal/ghr/... ./internal/githubsync/... ./cmd/ghreplica/...
• go test ./...
• go vet ./...
• go build ./cmd/ghreplica && go build ./cmd/ghr
• npm run docs:check

[dutifulbob]

Shipped to production on 5acd4f3986a7b93be787b9a6130516c494781ad2.

Validation:

• go test ./internal/searchindex/... ./internal/httpapi/... ./internal/ghr/... ./internal/githubsync/... ./cmd/ghreplica/... passed
• go test ./... passed
• go vet ./... passed
• go build ./cmd/ghreplica && go build ./cmd/ghr passed
• npm run docs:check passed
• codex review --base main ran with no actionable findings/comments
• PR CI passed: https://github.com/dutifuldev/ghreplica/actions/runs/24473320848
• merged main CI passed: https://github.com/dutifuldev/ghreplica/actions/runs/24473401492
• production deploy completed and /healthz + /readyz passed

Live production search validation against openclaw/openclaw:

• API matrix: 30/30 searches passed across fts, fuzzy, and regex
• CLI matrix: 15/15 searches passed across fts, fuzzy, and regex
• empty-result behavior verified for API and CLI

Representative successful queries:

• fts: heartbeat watchdog, thread binding, retry acp, heartbeat disabled
• fuzzy: watch dog, authstate, tele gram, matter most, greptil
• regex: auth.*state, tele.*gram, retry.*acp, heartbeat.*disabled

This PR specifically narrowed fuzzy and regex candidate sets so live search no longer times out on broad repo scans.