refactor: group use cases into DDD bounded contexts

Flat use_cases/ (44 files) + monolithic commands.rs/queries.rs
split into diary/, movies/, watchlist/, import/, auth/, users/,
integrations/, search/, person/, federation/ — each with own
commands.rs, queries.rs, and use case modules.

Inline tests extracted to sibling tests/ dirs.

crates/application/src/auth/login.rs

@@ -0,0 +1,45 @@
+use chrono::{DateTime, Utc};
+use uuid::Uuid;
+
+use domain::{errors::DomainError, value_objects::Email};
+
+use crate::{auth::queries::LoginQuery, context::AppContext};
+
+pub struct LoginResult {
+    pub token: String,
+    pub user_id: Uuid,
+    pub email: String,
+    pub expires_at: DateTime<Utc>,
+}
+
+pub async fn execute(ctx: &AppContext, query: LoginQuery) -> Result<LoginResult, DomainError> {
+    let email = Email::new(query.email)?;
+    let user = ctx
+        .repos
+        .user
+        .find_by_email(&email)
+        .await?
+        .ok_or_else(|| DomainError::Unauthorized("Invalid credentials".into()))?;
+
+    let valid = ctx
+        .services
+        .password_hasher
+        .verify(&query.password, user.password_hash())
+        .await?;
+    if !valid {
+        return Err(DomainError::Unauthorized("Invalid credentials".into()));
+    }
+
+    let generated = ctx.services.auth.generate_token(user.id()).await?;
+
+    Ok(LoginResult {
+        token: generated.token,
+        user_id: user.id().value(),
+        email: user.email().value().to_string(),
+        expires_at: generated.expires_at,
+    })
+}
+
+#[cfg(test)]
+#[path = "tests/login.rs"]
+mod tests;