Runtime feature toggles via admin server

[libotony]

Description

Adds a runtime feature-gate framework on the admin server, with three operational toggles (apilogs, txpool-api, pprof) flippable without restart.

Motivation: a recent incident required enabling pprof on a live prd node to investigate a sustained allocation spike — currently impossible without restart.

What's new

• GET/POST /admin/features/{name} — unified toggle with optional ttlSeconds (≤3600) auto-disable.
• apilogs, txpool-api, pprof registered; /admin/apilogs kept as deprecated alias.
• pprof handlers (/debug/pprof/*) move from the business API server to the admin server, default off.
• New metrics: admin_toggle_count{feature,to} and txpool_all_tx_count.

Breaking changes

• /node/txpool* returns 503 instead of 404 when disabled.
• --pprof alone no longer exposes pprof; needs --enable-admin. Startup warns otherwise.
• APIConfig.PprofOn removed; api/admin/apilogs Go package removed (HTTP alias preserved).

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• New and existing E2E tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have not added any vulnerable dependencies to my code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 75.37688% with 49 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cmd/thor/main.go	0.00%	26 Missing ⚠️
api/admin/featuregate/featuregate.go	84.00%	11 Missing and 9 partials ⚠️
cmd/thor/utils.go	0.00%	2 Missing ⚠️
cmd/thor/httpserver/admin_server.go	0.00%	1 Missing ⚠️

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