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, 28 insertions, 0 deletions

diff --git a/src/domain/error.rs b/src/domain/error.rs
new file mode 100644
index 0000000..4602b7e
--- /dev/null
+++ b/src/domain/error.rs
@@ -0,0 +1,28 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum DomainError {
+    #[error("Invalid time range: start time must be before end time")]
+    InvalidTimeRange,
+
+    #[error("Invalid color value")]
+    InvalidColor,
+
+    #[error("Invalid frequency")]
+    InvalidFrequency,
+
+    #[error("Invalid interval: must be greater than 0")]
+    InvalidInterval,
+
+    #[error("Calendar not found: {0}")]
+    CalendarNotFound(String),
+
+    #[error("Event not found: {0}")]
+    EventNotFound(String),
+
+    #[error("Recurrence not found: {0}")]
+    RecurrenceNotFound(String),
+
+    #[error("Cannot modify archived calendar")]
+    CalendarArchived,
+}