feat: nest API under /api/v1, structured error envelope, paged lists

Move every handler from /api/* to /api/v1/*. /api/* is now reserved for
future versioning.

Standardise the error response shape across the API as
{"error": {"code": "snake_case", "message": "..."}}. AppError gains a
`code()` whose top-level variants are matched exhaustively without a
wildcard — new variants are a compile error until coded. 500-class
responses always emit the fixed "internal error" string and log the
real cause via tracing only.

Lock in the list pagination envelope as {"items": [...], "page": {
"limit", "offset", "total"}} and apply it to GET /api/v1/mangas. `total`
serialises as null until feat/list-search-polish lands an indexed count.

The frontend client parses the envelope into ApiError.code with an
http_error fallback for non-JSON bodies. listMangas now returns the
paged shape; the root route consumes .items. New client.test.ts covers
envelope parsing and the fallback paths.

Lockstep version bump to 0.2.0.

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

backend/src/app.rs

@@ -30,7 +30,7 @@ pub async fn build(config: Config) -> anyhow::Result<Router> {
 /// so they can swap in a test DB pool and a `tempfile`-backed storage.
 pub fn router(state: AppState) -> Router {
     Router::new()
-        .nest("/api", crate::api::routes())
+        .nest("/api/v1", crate::api::routes())
         .with_state(state)
         .layer(TraceLayer::new_for_http())
 }