SCIX-897 feat: reconcile rate limiting and 429 error handling

[thostetler]

The node edge middleware's IP rate limiter redirected rate-limited users to the
home page with a generic toast, so its 429s and the upstream API gateway's 429s
took different code paths and no caller ever saw a real HTTP 429. Users got
inconsistent feedback and no path to a higher limit.

• Middleware returns a real HTTP 429 with Retry-After instead of redirecting: a
  minimal HTML page for document navigations, JSON for XHRs
• Node and upstream 429s flow through one client path: a toast for background
  requests and an inline SearchErrorAlert for foreground queries
• 429 copy encourages account creation, but only for anonymous users;
  authenticated users and the auth-unaware parseAPIError get a plain message
• Exempted the auth routes and /api/user from the node limiter so the
  account-creation link stays reachable when rate limited
• Stopped retrying 429 search requests; React Query stays retry:false
• Unified the 429 wording across the toast, inline alert, and full-page

Authenticated:

Anonymous:

[codecov]

Codecov Report

❌ Patch coverage is 80.68182% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.0%. Comparing base (2be2c28) to head (a4d0453).

Files with missing lines	Patch %	Lines
src/components/SolrErrorAlert/SolrErrorAlert.tsx	46.9%	17 Missing ⚠️
src/api/api.ts	55.6%	8 Missing ⚠️
src/store/store.ts	64.3%	5 Missing ⚠️
src/api/search/search.ts	50.0%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #901     +/-   ##
========================================
+ Coverage    67.0%   67.0%   +0.1%     
========================================
  Files         346     347      +1     
  Lines       41042   41410    +368     
  Branches     2084    2132     +48     
========================================
+ Hits        27489   27744    +255     
- Misses      13508   13621    +113     
  Partials       45      45             

Files with missing lines	Coverage Δ
src/lib/useSolrError.ts	75.8% <100.0%> (ø)
src/middleware.ts	92.4% <100.0%> (+0.9%)	⬆️
src/rateLimit.ts	47.0% <100.0%> (+3.5%)	⬆️
src/store/slices/notification.ts	90.3% <100.0%> (+0.5%)	⬆️
src/utils/common/parseAPIError.ts	100.0% <100.0%> (ø)
src/api/search/search.ts	56.1% <50.0%> (-0.1%)	⬇️
src/store/store.ts	46.7% <64.3%> (+2.1%)	⬆️
src/api/api.ts	82.9% <55.6%> (-1.3%)	⬇️
src/components/SolrErrorAlert/SolrErrorAlert.tsx	37.0% <46.9%> (ø)

... and 3 files with indirect coverage changes

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