feat(domain): add core domain models for calendar application

Implement domain-driven design layer with entities, value objects, and
repository traits for calendar management system.

Domain Entities:
- Calendar: manages calendar lifecycle and archival state
- Event: handles single occurrence events with cancellation support
- RecurringEvent: supports recurring events with exception handling
- RecurrenceException: manages modifications to specific occurrences

Value Objects:
- TimeRange: enforces valid start/end time constraints
- EventColor: type-safe color representation
- Frequency: recurrence frequency enumeration (daily/weekly/monthly/yearly)
- RecurrenceRule: encapsulates recurrence pattern logic
- CalendarId & EventId: essentially just a wrapper

Repository Traits:
- CalendarRepository: calendar persistence interface
- EventRepository: event querying and persistence with overlap detection
- RecurrenceRepository: recurring event management

Key Design Decisions:
- Use Uuid for entity IDs (type safety, performance)
- Encapsulate business logic within entities
- Immutable value objects with validation
- Repository pattern for infrastructure abstraction
- Clear separation of concerns between domain and persistence layers

All entities include timestamp tracking and follow builder pattern for
construction with validation at domain boundaries.

1 files changed, 7 insertions, 0 deletions

diff --git a/src/domain/mod.rs b/src/domain/mod.rs
new file mode 100644
index 0000000..05226bf
--- /dev/null
+++ b/src/domain/mod.rs
@@ -0,0 +1,7 @@
+pub mod calendar;
+pub mod event;
+pub mod recurrence;
+pub mod value_objects;
+pub mod repository;
+
+pub mod error;