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, 108 insertions, 0 deletions

diff --git a/src/domain/calendar.rs b/src/domain/calendar.rs
new file mode 100644
index 0000000..9e6a7fe
--- /dev/null
+++ b/src/domain/calendar.rs
@@ -0,0 +1,108 @@
+use std::collections::BTreeMap;
+
+use chrono::{DateTime, NaiveDate, Utc};
+use getset::Getters;
+use uuid::Uuid;
+
+use crate::domain::Event;
+
+#[derive(Clone, Debug, Getters)]
+#[getset(get = "pub")]
+pub struct Calendar {
+    id: Uuid,
+    name: String,
+    description: Option<String>,
+    events: BTreeMap<NaiveDate, Vec<Event>>,
+    is_archived: bool,
+    created_at: DateTime<Utc>,
+    updated_at: DateTime<Utc>,
+}
+
+pub struct CalendarBuilder {
+    name: String,
+    description: Option<String>,
+    events: BTreeMap<NaiveDate, Vec<Event>>,
+    is_archived: bool,
+}
+
+impl CalendarBuilder {
+    pub fn new(name: String) -> Result<Self, String> {
+        if name.is_empty() {
+            Err("Name cannot be empty".to_string())
+        } else {
+            Ok(Self {
+                name,
+                description: None,
+                events: BTreeMap::new(),
+                is_archived: false
+            })
+        }
+    }
+
+    pub fn description(mut self, description: Option<String>) -> Self {
+        self.description = description;
+        self
+    }
+
+    pub fn events(mut self, events: BTreeMap<NaiveDate, Vec<Event>>) -> Self {
+        self.events = events;
+        self
+    }
+    
+    pub fn is_archived(mut self, is_archived: bool) -> Self {
+        self.is_archived = is_archived;
+        self
+    }
+
+    pub fn build(self) -> Calendar {
+        let now = Utc::now();
+        Calendar {
+            id: Uuid::new_v4(),
+            name: self.name,
+            description: self.description,
+            events: self.events,
+            is_archived: self.is_archived,
+            created_at: now,
+            updated_at: now
+        }
+    }
+}
+
+impl Calendar {
+    pub fn add_events(&mut self, events: Vec<Event>) {
+        for event in events {
+            let date = event.start().date_naive();
+
+            self.events
+                .entry(date)
+                .or_insert_with(Vec::new)
+                .push(event);
+        }
+
+        self.touch()
+    }
+
+    pub fn update_name(&mut self, name: String) -> Result<(), String> {
+        if name.is_empty() {
+            Err("Name cannot be empty".to_string())
+        } else {
+            self.name = name;
+            self.touch();
+            Ok(())
+        }
+    }
+
+    pub fn update_description(&mut self, description: Option<String>) {
+        self.description = description;
+        self.touch()
+    }
+
+    pub fn update_archived(&mut self, is_archived: bool) {
+        self.is_archived = is_archived;
+        self.touch()
+    }
+
+    pub fn touch(&mut self) {
+        self.updated_at = Utc::now()
+    }
+}