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/storage/mod.rs

@@ -0,0 +1,31 @@
+//! Pluggable blob storage.
+//!
+//! Handlers depend on the `Storage` trait, never on a concrete backend.
+//! Add new backends (S3, GCS, …) as new impls in this module and wire
+//! them up in `app::build` based on config.
+
+mod local;
+
+use std::io;
+
+use async_trait::async_trait;
+
+pub use local::LocalStorage;
+
+#[derive(thiserror::Error, Debug)]
+pub enum StorageError {
+    #[error(transparent)]
+    Io(#[from] io::Error),
+    #[error("not found")]
+    NotFound,
+    #[error("invalid storage key")]
+    BadKey,
+}
+
+#[async_trait]
+pub trait Storage: Send + Sync {
+    async fn put(&self, key: &str, bytes: &[u8]) -> Result<(), StorageError>;
+    async fn get(&self, key: &str) -> Result<Vec<u8>, StorageError>;
+    async fn delete(&self, key: &str) -> Result<(), StorageError>;
+    async fn exists(&self, key: &str) -> Result<bool, StorageError>;
+}