feat: add Redis-backed cache proxy for search and resolver endpoints

[thostetler]

Client-side search and resolver requests were hitting the upstream API
directly on every page load with no server-side caching layer. This
adds a Redis-backed cache proxy that sits between the browser and the
upstream Solr/resolver APIs.

• Add /api/proxy/[...path] catch-all route with session validation, origin check, rate limiting, and path allowlist
• Add Redis singleton (ioredis) with lazy init, health tracking, and graceful degradation
• Add cache-aside pattern: check Redis first, forward upstream on miss, cache 2xx responses
• Wire client-side search and resolver hooks to route through the proxy
• Flatten array query params (fl, sort) to comma-separated strings for Solr compatibility
• Cache key format: scix:cache:GET:/path?sorted-params with 5min TTL
• Add 27 unit tests covering proxy handler, cache key builder, param flattener, and Redis singleton

[codecov]

Codecov Report

❌ Patch coverage is 69.75477% with 111 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.6%. Comparing base (968d8a6) to head (8f24b3a).

Files with missing lines	Patch %	Lines
src/pages/api/proxy/[...path].ts	68.8%	43 Missing and 1 partial ⚠️
src/api/search/search.ts	16.7%	25 Missing ⚠️
src/lib/serverside/absCanonicalization.ts	66.7%	25 Missing ⚠️
src/lib/redis.ts	79.7%	13 Missing ⚠️
src/api/resolver/resolver.ts	71.5%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #805     +/-   ##
========================================
+ Coverage    61.6%   61.6%   +0.1%     
========================================
  Files         312     315      +3     
  Lines       36000   36320    +320     
  Branches     1594    1649     +55     
========================================
+ Hits        22148   22355    +207     
- Misses      13815   13927    +112     
- Partials       37      38      +1     

Files with missing lines	Coverage Δ
src/lib/proxy-cache.ts	100.0% <100.0%> (ø)
src/api/resolver/resolver.ts	74.5% <71.5%> (-22.7%)	⬇️
src/lib/redis.ts	79.7% <79.7%> (ø)
src/api/search/search.ts	43.5% <16.7%> (-1.6%)	⬇️
src/lib/serverside/absCanonicalization.ts	60.7% <66.7%> (-2.4%)	⬇️
src/pages/api/proxy/[...path].ts	68.8% <68.8%> (ø)

... and 2 files with indirect coverage changes

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