Capture search query text in searches.db for top + zero-result analytics

[ptrlrd]

Summary

Adds persistent search-query logging so we can see what people search for (not just the count by entity type that Prometheus tracks today). New searches.db lives next to runs.db inside DATA_DIR, so the existing Litestream pipeline covers it for free.

What ships

• services/searches_db.py — schema (searches table + 3 indexes), background writer thread with a 10k-row bounded queue (drops on overflow, never blocks the request), helpers for top_searches / recent_searches / search_volume. IP is hashed with a daily UTC salt so the same client de-dupes within a day but isn't trackable across days.
• Middleware hook in main.py — the existing ?search= detection branch now also enqueues (query, entity_type, lang, ip_hash) alongside the existing Prometheus counter increment.
• routers/admin_searches.py — gated by X-Admin-Token header (sourced from ADMIN_TOKEN env). Three endpoints:
  • GET /api/admin/searches/top?days=7&limit=100 — most-searched queries
  • GET /api/admin/searches/recent?limit=200 — newest first, spot-check live traffic
  • GET /api/admin/searches/volume?days=30 — per-day rollup for trends
• Compose files — ADMIN_TOKEN plumbed through both prod + beta.

Why a separate DB

Writes are ~2/sec sustained, way higher than runs.db write rate. A separate file isolates the workloads so search write bursts don't compete with run submission/leaderboard reads, and the table can be wiped/retention-trimmed independently without touching run data.

After merge

1. Set ADMIN_TOKEN=<random> in .env on the server (anything missing/empty returns 503 from the admin endpoints — explicit "not configured" instead of silent failures).
2. Once deployed, query:

   curl -H "X-Admin-Token: $ADMIN_TOKEN" https://spire-codex.com/api/admin/searches/top?days=7
   

3. The zero-result view requires the search endpoints to know whether they returned 0 hits — not in this PR. Top + recent + volume work immediately on any traffic that hits a list endpoint with ?search=.