feat(routes-f): add weighted featured channel rotation endpoint and t…

[Francis6-git]

Description:
This PR addresses issue #1011 by adding a completely self-contained deterministic, weighted featured channel slot selector under app/api/routes-f/featured-channel/.

The system leverages an in-memory rotation manager driven by a seeded PRNG (mulberry32) to achieve perfect determinism against any arbitrary string-based rotation_id, while respecting candidate selection weights.

Changes Covered:

• GET Endpoint: Returns the currently active featured_creator, rotation_id, and a future rotates_at ISO date string.
• Candidate Array: Bundles exactly 20 rich, platform-specific mock creator profiles (wallet_address, category, followers, is_live, and selection weight).
• Deterministic-by-ID: Employs string hashing mapped directly into an integer PRNG fallback chain, guaranteeing the exact same candidate returns for identical rotation_id strings.
• POST Override: Exposes a manual transition path at /api/routes-f/featured-channel (via Next.js dynamic routing setups on single file multi-methods) advancing the sequential global rotation index state.
• Test Verification: Tests validation, distributions, field schema constraints, and determinism.

Closes #1011

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