feat(indexer): redundant RPC provider switching with cool-down (#151)

[magaret457]

Summary

Closes #151

Implements redundant RPC provider failover for the indexer, resolving #151.

Changes

rpc.rs

• Introduces ProviderManager — a thread-safe (Arc<RwLock<>>) prioritised list of RPC URLs.
• healthy_url() walks the list and returns the first non-failed provider. If a provider's cool-down has expired it is automatically re-enabled.
• mark_failed(url) stamps a provider with the current Instant; it stays out of rotation for RPC_COOLDOWN_SECS (default 60 s).
• fetch_events now accepts &ProviderManager instead of a bare &str. On a 5xx, 429, or connection error it calls mark_failed and loops immediately to the next provider — zero added latency on the happy path.
• When all providers are unhealthy, exponential back-off (2 s → 60 s) kicks in and a error! log is emitted.

config.rs

• New fields: rpc_fallback_urls: Vec<String> (from RPC_FALLBACK_URLS, comma-separated) and rpc_cooldown_secs: u64 (from RPC_COOLDOWN_SECS, default 60).

indexer.rs

• IndexerState gains a providers: ProviderManager field.
• poll_once receives &ProviderManager and passes it to fetch_events.

main.rs

• Constructs ProviderManager from config and injects it into IndexerState.

backend/indexer/.env.example (new)

• Documents all environment variables including the new RPC failover ones.

Failover Strategy

Primary OK → use primary (no overhead)
Primary 5xx/429/timeout → mark_failed(primary), try fallback-1 immediately
All failed → exponential back-off + critical log, retry until one recovers
Cool-down expires → provider silently re-enters rotation on next healthy_url() call

Testing

• Existing unit tests in rpc.rs are unaffected (they test decode/parse logic, not the HTTP layer).
• To manually test failover: set RPC_URL to an invalid endpoint and RPC_FALLBACK_URLS to a valid one — the indexer should switch within one poll cycle.

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[ayomideadeniran]

Thanks for the PR! This is currently under review and I will get back to you shortly.