Implement anonymised analytics event model

Use freezed for immutability and code generation — this eliminates manual copyWith and == boilerplate. Annotate with @JsonSerializable(explicitToJson: true) to ensure nested ExpenseType and ExpenseTypePair serialise correctly. For the PII lint rule, the simplest approach is a custom dart_code_metrics or custom_lint rule that checks field names against a blocklist; alternatively document it as a code review checklist item enforced in CI. Generate sessionId with the uuid package (Uuid().v4()) at the call site (in the tracker), not inside the model constructor — keeping the model a pure data container makes it easier to test with known session IDs.

Use DateTime.now().millisecondsSinceEpoch for timestampMs at event creation time.

Unit tests covering: construction of all four eventType variants, JSON round-trip for each variant (including null blockedPair and populated blockedPair), assertion that sessionId is a valid v4 UUID format (regex check), assertion that selectedTypes serialises as string keys not integers, and a PII-field presence check (assert no field named userId/email/name etc. exists via reflection or manual check). Use flutter_test. No async or platform dependencies.