Bring back the network Leaderboard, rebuilt around per repository emissions and dual track eligibility

[Dexterity104]

Summary

The network Leaderboard was deleted in #1220 when the per repository eligibility miner API shape landed. The old page modelled eligibility as a single network wide flag, ranked on one combined column, and had no concept of an issues treasury, a per repo open PR slot ceiling, or a failed_reason penalization gate. Keeping it live would have lied about the policy, so /top-miners has been redirecting to /repositories ever since as a holding pattern.

The validator has not stood still. Repository scoped emissions (gittensor#1235), per repo eligibility (gittensor#1293), per repo scoring config (gittensor#1300), the maintainer carve out (gittensor#1292), collateral inside repo allocation (gittensor#1272), and per repo min_token_score honor on the PR scoring path (gittensor#1330) have all shipped since.

Problems

• There is no network wide surface at all. The only ranking views left are the per repository miners tab and the watchlist, neither of which answers "how is the network as a whole doing right now".
• The 90/10 emission split (OSS_EMISSION_SHARE = 0.90 for miners and ISSUES_TREASURY_EMISSION_SHARE = 0.10 flat to ISSUES_TREASURY_UID = 111) has no representation anywhere in the app, even though it sets the upper bound on what a miner can ever earn.
• Dual track eligibility is invisible at the network level. A miner cannot tell which cohort the validator places them in (eligible in both tracks, OSS only, Discovery only, scoring but below threshold, or fully inactive).
• The per repo open PR slot ceiling (min(base + floor(token_score / per_slot), max)) has no visible counterpart. Miners hit the spam threshold without warning.
• failed_reason based penalization is hidden. A penalized miner looks identical to a healthy one in every list.
• The review quality penalty (multiplier = max(0, 1 − 0.15 × N)) is opaque. Miners see a reduced score with no way to count the maintainer CHANGES_REQUESTED rounds behind it.
• Rank movement is unknowable. Without a snapshot anywhere in the app, climbs and slips between rounds are invisible.
• External links pointing at /top-miners keep bouncing off /repositories. The redirect was meant to be temporary.

Goal

Bring the Leaderboard back at /leaderboard, fully rebuilt against the policy the validator actually runs today, and held to the quality bar set by #95. Full design, components, and policy mapping are covered in the PR.