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, 76 insertions, 0 deletions

diff --git a/src/domain/calendar.rs b/src/domain/calendar.rs
new file mode 100644
index 0000000..21216b5
--- /dev/null
+++ b/src/domain/calendar.rs
@@ -0,0 +1,76 @@
+use chrono::{DateTime, Utc};
+use getset::Getters;
+
+use crate::domain::value_objects::CalendarId;
+
+#[derive(Debug, Clone, Getters)]
+pub struct Calendar {
+    #[getset(get = "pub")]
+    id: CalendarId,
+    #[getset(get = "pub")]
+    name: String,
+    #[getset(get = "pub")]
+    description: Option<String>,
+    #[getset(get = "pub")]
+    is_archived: bool,
+    #[getset(get = "pub")]
+    created_at: DateTime<Utc>,
+    #[getset(get = "pub")]
+    updated_at: DateTime<Utc>,
+}
+
+impl Calendar {
+    pub fn new(name: String, description: Option<String>) -> Self {
+        let now = Utc::now();
+        Self {
+            id: CalendarId::new(),
+            name,
+            description,
+            is_archived: false,
+            created_at: now,
+            updated_at: now,
+        }
+    }
+
+    pub fn with_id(
+        id: CalendarId,
+        name: String,
+        description: Option<String>,
+        is_archived: bool,
+        created_at: DateTime<Utc>,
+        updated_at: DateTime<Utc>,
+    ) -> Self {
+        Self {
+            id,
+            name,
+            description,
+            is_archived,
+            created_at,
+            updated_at,
+        }
+    }
+
+    pub fn archive(&mut self) {
+        self.is_archived = true;
+        self.touch();
+    }
+
+    pub fn unarchive(&mut self) {
+        self.is_archived = false;
+        self.touch();
+    }
+
+    pub fn update_name(&mut self, name: String) {
+        self.name = name;
+        self.touch();
+    }
+
+    pub fn update_description(&mut self, description: Option<String>) {
+        self.description = description;
+        self.touch();
+    }
+
+    pub fn touch(&mut self) {
+        self.updated_at = Utc::now();
+    }
+}