feat: decode contract event logs using verified ABIs

[pthmas]

Summary

Adds a background worker that decodes EVM contract event logs using verified contract ABIs (including proxy contracts). Decoded results are persisted and surfaced through the API.

Changes

• contract_abi.rs: Resolves ABIs for contracts — direct or via EIP-1967/EIP-1822 proxy storage slots (calls eth_getStorageAt to find implementation address)
• event_log_decode.rs: Core decoding logic — matches topic0 to ABI events, decodes indexed/non-indexed params, serializes results; defines decode statuses (pending, decoded, no_abi, no_matching_event, decode_failed)
• indexer/event_log_decode_worker.rs: Background worker — claims jobs from event_log_decode_jobs queue (SKIP LOCKED), processes logs in batches of 500 per address, supports full rescans
• Migration 20260528000001: Adds decode_status, decoded_at, decode_attempted_at, decode_source columns to event_logs; creates event_log_decode_jobs queue table; adds partial index on pending logs
• api/handlers/logs.rs: Simplified — strips proxy forwarding logic, returns decoded fields in log responses
• api/handlers/proxy.rs: Cleaned up — removes log-related proxy passthrough now handled natively
• api/handlers/contracts.rs: Queues a decode job when a contract ABI is uploaded
• indexer/indexer.rs + main.rs: Spawns EventLogDecodeWorker as a tokio task alongside existing workers
• frontend/src/types/index.ts: Adds EventLog type fields for decode status and decoded params
• tests/integration/logs.rs: Integration tests for log API responses with decoded data

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