chore: initial project scaffold

Set up Mangalord with a Rust/axum backend, SvelteKit frontend, Postgres,
and Docker Compose deployment. Establishes the architecture and TDD
patterns the project will extend:

- Hexagonal-ish backend layering (domain / repo / storage / api) with
  a pluggable Storage trait (LocalStorage today, S3 as a future impl).
- Initial migration: users, mangas, chapters, bookmarks.
- Vertical slice for mangas (list, search, create, get) with
  #[sqlx::test] integration coverage and storage unit tests.
- SvelteKit frontend using Svelte 5 runes, typed API client, Vitest
  unit tests and Playwright e2e with route mocking.
- CLAUDE.md documenting layering, TDD/git/SemVer workflow rules, and
  extension points (tags, fulltext search, OCR, S3, auth).
- Project-scoped .claude/settings.json with permission allowlist for
  the toolchain (git, cargo, npm/vite, docker, psql, gh, doc fetches).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/src/error.rs

@@ -0,0 +1,45 @@
+use axum::http::StatusCode;
+use axum::response::{IntoResponse, Response};
+use axum::Json;
+use serde_json::json;
+
+use crate::storage::StorageError;
+
+#[derive(thiserror::Error, Debug)]
+pub enum AppError {
+    #[error("not found")]
+    NotFound,
+    #[error("invalid input: {0}")]
+    InvalidInput(String),
+    #[error(transparent)]
+    Database(#[from] sqlx::Error),
+    #[error(transparent)]
+    Storage(#[from] StorageError),
+    #[error(transparent)]
+    Other(#[from] anyhow::Error),
+}
+
+pub type AppResult<T> = Result<T, AppError>;
+
+impl IntoResponse for AppError {
+    fn into_response(self) -> Response {
+        let (status, message) = match &self {
+            AppError::NotFound => (StatusCode::NOT_FOUND, self.to_string()),
+            AppError::InvalidInput(_) => (StatusCode::BAD_REQUEST, self.to_string()),
+            AppError::Database(sqlx::Error::RowNotFound) => {
+                (StatusCode::NOT_FOUND, "not found".to_string())
+            }
+            AppError::Storage(StorageError::NotFound) => {
+                (StatusCode::NOT_FOUND, "not found".to_string())
+            }
+            AppError::Storage(StorageError::BadKey) => {
+                (StatusCode::BAD_REQUEST, "invalid file key".to_string())
+            }
+            AppError::Database(_) | AppError::Storage(_) | AppError::Other(_) => {
+                tracing::error!(error = ?self, "internal error");
+                (StatusCode::INTERNAL_SERVER_ERROR, "internal error".to_string())
+            }
+        };
+        (status, Json(json!({ "error": message }))).into_response()
+    }
+}