Build audience-specific notification payload builders

Define FcmPayload as an immutable Dart class with a toJson() method that produces the FCM v1 message body structure. The FCM v1 format requires a 'message' wrapper with 'notification' (title/body) and 'data' (string key-value pairs) — ensure all data map values are explicitly cast to String since FCM rejects non-string data values. For the coordinator batching, use a groupBy utility (from the collection package) to group candidates by coordinatorId in a single pass before building payloads. Implement a _formatMentorNameShort(String firstName, String lastName) private helper that produces 'Ola N.' format — centralise PII truncation logic here.

The 4KB limit check should be implemented as an assert in debug mode and a logged warning (not throw) in release mode to avoid crashing the reminder pipeline over a marginal overage. Use separate builder methods (not a generic builder with an audience flag) for clarity — the two payloads have structurally different inputs.

Unit tests (flutter_test): (1) buildMentorReminderPayload with a 7-day candidate; assert body contains '7 days', deep_link routes to /certification/{certId}, data map contains threshold_days=7 and cert_type. (2) buildMentorReminderPayload with 14-day and 30-day candidates; assert correct day counts and threshold values. (3) buildCoordinatorReminderPayload with 60 candidates for one coordinator; assert body contains '… and X more' and total payload < 4KB. (4) Assert PII check: no '@' (email) and no digit sequences >5 chars (personnummer proxy) in any payload field.

(5) jsonEncode/jsonDecode round-trip test for both payload types. (6) buildCoordinatorReminderPayload with candidates for 3 different coordinators; assert Map contains 3 keys with correct grouping. (7) Empty candidate list to coordinator builder returns empty Map. Coverage target: 90% line coverage.