feat(api): add in_successful_call filter to GET /events (Resolves #211)

[teeschima]

Summary

Resolves #211 — adds ?in_successful_call=true|false to GET /events (and GET /contracts/{id}/events, which shares the filter parser) to filter diagnostic events from failed calls vs successful-call events.

What changed

• internal/store/store.go — EventFilter gains InSuccessfulCall *bool: nil means "no constraint", true/false filter explicitly. A pointer (not a plain bool) so "absent" stays distinguishable from false.
• internal/store/postgres.go — QueryEvents appends the parameterized clause in_successful_call = $N when the filter is set; unset filters keep the exact previous SQL.
• internal/api/handlers.go — filterFromQuery parses in_successful_call (true | false); any other value is a 400. Absent (or empty) means no constraint.
• internal/api/api_test.go — table-driven TestListEvents_InSuccessfulCallFilter (omitted / true / false) plus in_successful_call=maybe added to the invalid-params suite.
• internal/store/postgres_test.go — integration test against a real database: successful-only, failed-only, and nil (no constraint) queries, table-driven.
• README.md — the new parameter is documented in the GET /events API reference table.

Design notes

• *bool semantics match the convention the project later adopted upstream: nil = no filter, so existing requests behave byte-for-byte as before.
• No schema, config, or endpoint changes — the in_successful_call column already exists.

Testing

• go build ./..., go vet ./..., golangci-lint run, and go test ./... all pass, including the Postgres integration suite.

Closes #211