feat: customize rate limit per endpoint

[Bellamzy]

Here's a PR description you can use:

───────────────────────────────────────────────────────────────────────────────────────────────

feat: customize rate limit per endpoint

Replaces the single global rate limit with per-route tiers so different endpoint groups can be
throttled independently.

Changes:

• constants.rs — added four tier constants (burst + period) for Light, Read, User, and Write
  tiers
• rate_limit.rs (new) — RateLimitTier enum and with_rate_limit(router, tier) helper; no-op in
  test builds to prevent test cross-contamination
• routes/v1.rs — router() splits routes into four sub-routers, each wrapped with its tier
• server.rs — removed duplicate global GovernorLayer (rate limiting now owned by sub-routers)
• lib.rs — registered pub mod rate_limit

Tier breakdown:

┌───────┬────────────────────────────────────────────┬────────────────┐
│ Tier │ Endpoints │ Limit │
├───────┼────────────────────────────────────────────┼────────────────┤
│ Light │ /, /health, /fees, /prices, /ws │ 120 req / 60 s │
├───────┼────────────────────────────────────────────┼────────────────┤
│ Read │ /pools, /stats, /leaderboard, /referrals/* │ 60 req / 60 s │
├───────┼────────────────────────────────────────────┼────────────────┤
│ User │ /users/{address}/* │ 30 req / 60 s │
├───────┼────────────────────────────────────────────┼────────────────┤
│ Write │ /indexer/* │ 20 req / 60 s │
└───────┴────────────────────────────────────────────┴────────────────┘
closes #1146

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