fix: prevent SQLite lock churn and validate EVM RPC capabilities

[quqi1599]

Summary

This fixes two production failure modes that can silently reduce payment-recognition reliability in the default pure-Go SQLite deployment:

• repeated SQLITE_BUSY / database is locked errors while cleaning transaction locks and updating EVM scan cursors;
• RPC nodes being marked healthy when their TCP port is reachable but the required eth_getLogs or eth_subscribe capability is rate-limited or disabled.

It also makes live EVM WebSocket subscriptions start at latest. Without an explicit FromBlock, go-ethereum serializes the filter as fromBlock: 0x0, which some public providers reject as an oversized historical range.

Root cause

The pure-Go SQLite driver uses modernc.org/sqlite. Its per-connection PRAGMA syntax is _pragma=...; the previous _busy_timeout=5000 DSN option was ignored. Running PRAGMA busy_timeout=5000 once after opening the database only configured one pooled connection, while the runtime database allowed multiple concurrent writers.

RPC health checks only performed a TCP dial. That allowed endpoints returning HTTP 403/429 or WebSocket subscription quota errors to remain ok and be selected repeatedly by listeners and backfill scanners.

Changes

• apply WAL, synchronous=NORMAL, and a 5-second busy timeout to every pure-Go SQLite connection;
• serialize runtime SQLite writes through one connection;
• reuse the bounded SQLite busy retry for expired transaction-lock cleanup;
• verify EVM HTTP nodes with eth_blockNumber plus the scanner's cursor-based historical eth_getLogs filter;
• verify EVM WebSocket nodes by establishing the actual filtered log subscription;
• start ETH/BSC/Polygon/Plasma live subscriptions at latest;
• use capability-aware checks in the admin RPC health endpoint;
• replace the rate-limited default BSC WebSocket endpoint and add a working general HTTP backfill endpoint;
• add regression tests for per-connection busy timeout, cleanup retry, HTTP log capability, WebSocket subscription capability, and fromBlock: latest.

Validation

• go test ./...
• go vet ./...
• CGO_ENABLED=0 go build -trimpath .
• targeted -race runs for all newly added SQLite, retry, HTTP RPC, and WebSocket RPC tests

In a single-instance deployment, the change stopped new SQLite busy errors during the observation window, restored a continuously advancing BSC backfill cursor, and correctly classified quota-limited/unsupported RPC nodes as down.

No schema migration is introduced. A real on-chain payment was not executed as part of validation.