Improve premium search enforcement and Svelte upgrade guidance

[Copilot]

What changed

• Improved the Svelte event and stack search experience so free-plan users see upgrade guidance for the effective filter, including saved views.
• Made premium-field detection resource-specific so free stack fields such as critical, first, and last are not incorrectly labeled premium.
• Kept premium search enforcement at the API boundary for event search, event count, and stack search requests.

Why

The Svelte UI previously used the event field rules for both event and stack routes and only inspected the raw URL filter. This produced false upgrade prompts for valid free stack searches and missed premium filters supplied by saved views. Direct API requests could also bypass UI guidance and receive silently filtered results.

Behavior

• Premium search filters on non-premium organizations return 426 Upgrade Required.
• The Svelte UI shows upgrade guidance for premium event and stack filters.
• Free event and stack filters continue to work without an upgrade prompt.
• No public API shape, WebSocket message, configuration key, or legacy Angular behavior changed.

Verification

• npm run validate
• npm run test:unit — 310 passed
• npm run build
• Three targeted controller regressions — passed
• Local Svelte dogfood against Aspire:
  • stack critical:true — no upgrade prompt
  • stack title:timeout — upgrade prompt shown
  • event reference:ABC123 — no upgrade prompt
  • event tags:important — upgrade prompt shown

Breaking changes

None.

[CLAassistant]

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1 out of 2 committers have signed the CLA.

✅ niemyjski
❌ Copilot
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[chatgpt-codex-connector]

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[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
Exceptionless.AppHost	38%	39%	144	❌
Exceptionless.Core	72%	64%	8854	➖
Exceptionless.Insulation	23%	23%	205	❌
Exceptionless.Web	84%	67%	6866	✔
Summary	76% (21400 / 28173)	65% (9826 / 15182)	16069	✔