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, 45 insertions, 0 deletions

diff --git a/src/domain/repository.rs b/src/domain/repository.rs
new file mode 100644
index 0000000..00b3974
--- /dev/null
+++ b/src/domain/repository.rs
@@ -0,0 +1,45 @@
+use async_trait::async_trait;
+use super::{
+    calendar::Calendar,
+    event::Event,
+    recurrence::RecurringEvent,
+    value_objects::{CalendarId, EventId, TimeRange},
+};
+
+pub type Result<T> = std::result::Result<T, RepositoryError>;
+
+#[derive(Debug, thiserror::Error)]
+pub enum RepositoryError {
+    #[error("Entity not found")]
+    NotFound,
+    
+    #[error("Database error: {0}")]
+    DatabaseError(String),
+    
+    #[error("Constraint violation: {0}")]
+    ConstraintViolation(String),
+}
+
+#[async_trait]
+pub trait CalendarRepository: Send + Sync {
+    async fn save(&self, calendar: &Calendar) -> Result<()>;
+    async fn find_by_id(&self, id: &CalendarId) -> Result<Option<Calendar>>;
+    async fn find_all_active(&self) -> Result<Vec<Calendar>>;
+    async fn delete(&self, id: &CalendarId) -> Result<()>;
+}
+
+#[async_trait]
+pub trait EventRepository: Send + Sync {
+    async fn save(&self, event: &Event) -> Result<()>;
+    async fn find_by_id(&self, id: &EventId) -> Result<Option<Event>>;
+    async fn find_by_calendar(&self, calendar_id: &CalendarId) -> Result<Vec<Event>>;
+    async fn find_in_range(&self, calendar_id: &CalendarId, range: &TimeRange) -> Result<Vec<Event>>;
+    async fn delete(&self, id: &EventId) -> Result<()>;
+}
+
+#[async_trait]
+pub trait RecurringEventRepository: Send + Sync {
+    async fn save(&self, event: &RecurringEvent) -> Result<()>;
+    async fn find_by_calendar(&self, calendar_id: &CalendarId) -> Result<Vec<RecurringEvent>>;
+    async fn delete(&self, id: &str) -> Result<()>;
+}