feat(domain): Add initial domain models for calendar, event, and recurrenceHEADmaster

- Added `calendar.rs` with Calendar entity and builder
- Added `event.rs` with Event model and builder
- Added `recurrence.rs` for recurrence rules
- Added `mod.rs` to expose domain module structure

These files establish the core domain layer structures
for future business logic.

1 files changed, 36 insertions, 0 deletions

diff --git a/src/domain/recurrence.rs b/src/domain/recurrence.rs
new file mode 100644
index 0000000..807df4b
--- /dev/null
+++ b/src/domain/recurrence.rs
@@ -0,0 +1,36 @@
+use chrono::{DateTime, Utc};
+use getset::Getters;
+
+#[derive(Clone, Debug, Getters)]
+#[getset(get = "pub")]
+pub struct RecurrenceRule {
+    frequency: Frequency,
+    interval: u32,
+    end_date: Option<DateTime<Utc>>
+}
+
+#[derive(Clone, Debug)]
+pub enum Frequency {
+    DAILY,
+    WEEKLY,
+    MONTHLY,
+    YEARLY
+}
+
+impl RecurrenceRule {
+    pub fn new(
+        frequency: Frequency,
+        interval: u32,
+        end_date: Option<DateTime<Utc>>
+    ) -> Result<Self, String> {
+        let now = Utc::now();
+
+        if let Some(end) = end_date {
+            if now >= end {
+                return Err("End date has already passed".to_string());
+            }
+        }
+
+        Ok(Self { frequency, interval, end_date })
+    }
+}