Implement Report Schema Service

Use a Repository pattern: ReportSchemaService depends on IReportSchemaCache and IOrgFieldConfigLoader interfaces, not concrete implementations — this makes unit testing with mockito straightforward. Store the schema version hash alongside the cached schema so version comparison is a simple string equality check. For background refresh, use Future.microtask() or unawaited() with proper error swallowing (log, do not propagate). Define a ReportSchema sealed class (or freezed union) with a version field so downstream BLoC can react to version changes via stream equality.

Ensure Riverpod provider is defined with autoDispose if schemas are org-scoped to prevent memory leaks when switching orgs. Avoid storing org-field-config-loader raw responses in cache — always store the parsed ReportSchema to keep the cache layer typed.

Unit tests with flutter_test and mockito covering: (1) cache-hit returns cached schema without calling loader, (2) cache-miss calls loader and stores result, (3) TTL expiry triggers re-fetch, (4) schema version mismatch invalidates cache, (5) org A schema does not appear for org B (isolation), (6) malformed JSON from loader returns last valid cached schema, (7) malformed JSON with no cache throws SchemaParseException. Mock both org-field-config-loader and report-schema-cache. Target 90%+ line coverage. No integration tests required for this task — covered by task-007.