feat: full clean architecture refactor — domain → application → infrastructure → presentation

- Restructure from Modular MVC to Clean Architecture with 4 strict layers
- Domain: entities (anime/komik/proxy), Repository traits, typed errors
- Application: use case classes with proper error propagation
- Infrastructure: repository impls, parsers, Redis cache, HTTP/scraping, browser
- Presentation: Axum handlers, DTOs, AppState, router, AppError+IntoResponse
- Migrate all parsers (otakudesu, alqanime, komik) to native infra implementations
- Replace once_cell::sync::Lazy/OnceCell with std::sync::LazyLock/OnceLock
- Remove 150+ old files in modules/ and shared/ directories
- Remove once_cell from Cargo.toml dependencies
- Fix test/debug binaries to use new import paths
- Zero new clippy warnings

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/infrastructure/scraping/retry.rs

@@ -0,0 +1,66 @@
+//! HTTP retry utilities with exponential backoff.
+
+use backoff::ExponentialBackoff;
+use std::time::Duration;
+
+/// Default retry configuration for HTTP requests.
+pub fn default_backoff() -> ExponentialBackoff {
+    ExponentialBackoff {
+        initial_interval: Duration::from_millis(500),
+        max_interval: Duration::from_secs(10),
+        multiplier: 2.0,
+        max_elapsed_time: Some(Duration::from_secs(30)),
+        ..Default::default()
+    }
+}
+
+/// Create a custom exponential backoff.
+pub fn custom_backoff(
+    initial_ms: u64,
+    max_secs: u64,
+    multiplier: f64,
+    max_elapsed_secs: u64,
+) -> ExponentialBackoff {
+    ExponentialBackoff {
+        initial_interval: Duration::from_millis(initial_ms),
+        max_interval: Duration::from_secs(max_secs),
+        multiplier,
+        max_elapsed_time: Some(Duration::from_secs(max_elapsed_secs)),
+        ..Default::default()
+    }
+}
+
+/// Quick backoff for fast retries (3 attempts, 100ms initial).
+pub fn quick_backoff() -> ExponentialBackoff {
+    ExponentialBackoff {
+        initial_interval: Duration::from_millis(100),
+        max_interval: Duration::from_secs(1),
+        multiplier: 2.0,
+        max_elapsed_time: Some(Duration::from_secs(5)),
+        ..Default::default()
+    }
+}
+
+/// Slow backoff for long operations (10 attempts, 1s initial).
+pub fn slow_backoff() -> ExponentialBackoff {
+    ExponentialBackoff {
+        initial_interval: Duration::from_secs(1),
+        max_interval: Duration::from_secs(30),
+        multiplier: 2.0,
+        max_elapsed_time: Some(Duration::from_secs(120)),
+        ..Default::default()
+    }
+}
+
+/// Make an error transient (will be retried).
+pub fn transient<E>(err: E) -> backoff::Error<E> {
+    backoff::Error::transient(err)
+}
+
+/// Make an error permanent (will NOT be retried).
+pub fn permanent<E>(err: E) -> backoff::Error<E> {
+    backoff::Error::permanent(err)
+}
+
+// Re-export retry function for convenience
+pub use backoff::future::retry;