[Refactor] Consolidate deployment experiment runtime state

[roomote-roomote]

​Created by Roomote. View the task or mention @roomote-roomote for follow-up asks.

What changed

• Consolidated deployment experiment audience, metadata-key, and member-readable runtime eligibility into one typed feature-flag descriptor.
• Replaced the feature-specific nightly runtime procedures with one parameterized runtime read and one shared client hook for Dizzy and Auto tool approvals.
• Replaced hard-coded runtime invalidation branches with descriptor-driven invalidation while retaining a single query for the selected customer-preview or internal-nightly audience.

Why this change was made

PR #3190 and PR #3207 left audience selection, runtime eligibility, and cache invalidation split across separate contracts. The shared descriptor makes adding another nightly experiment a typed classification instead of another feature-specific query or invalidation branch.

Impact

Admin-only management reads and writes remain separated between customer-preview and internal-nightly audiences. Member-readable runtime reads still require the server-provided nightly deployment opt-in, and server-side experiment checks still enforce R_NIGHTLY_EXPERIMENTS_ENABLED; nightly settings without a runtime surface remain unavailable to members. Focused feature-flags, database, web client, and web server tests passed, as did full lint, typecheck, and pre-push checks. The full web test run passed all client tests but retained eight existing environment-sensitive dev-login failures; browser proof is not applicable to this non-visual contract refactor.

[roomote-community]

No code issues found. See task

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