refactor: TypedHandler[T] — typed params for all task handlers

[bdchatham]

Summary

Introduces engine.TypedHandler[T] — a generic helper that gives every task handler compile-time typed params without changing the engine, store, server, or client.

All 17 handlers converted. Manual parse functions eliminated. Net -216 lines.

How it works

// Before: fragile type assertions
func (r *SnapshotRestorer) Handler() engine.TaskHandler {
    return func(ctx context.Context, params map[string]any) error {
        bucket, _ := params["bucket"].(string)  // silent zero on mismatch
        ...
    }
}

// After: typed params with JSON struct tags
func (r *SnapshotRestorer) Handler() engine.TaskHandler {
    return engine.TypedHandler(func(ctx context.Context, cfg SnapshotConfig) error {
        if cfg.Bucket == "" { return fmt.Errorf("missing bucket") }
        return r.Restore(ctx, cfg)
    })
}

What's removed

All manual parse functions: parseSnapshotConfig, parseUploadConfig, parseExportConfig, parseAssembleConfig, parseNodeNames, parseArtifactUploadConfig, parseGenesisPeersConfig, parseGenesisS3Config, intentFromParams, extractStringMap, parseSources, parseEC2TagsSource, parseStaticSource, toStringMap, toStringSlice, parseTargetHeight

What's unchanged

Engine, SQLite store, HTTP server, client, OpenAPI spec, serve.go registration.

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