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/api/mangas.rs

@@ -4,6 +4,7 @@ use axum::{Json, Router};
 use serde::Deserialize;
 use uuid::Uuid;
 
+use crate::api::pagination::PagedResponse;
 use crate::app::AppState;
 use crate::domain::manga::{Manga, NewManga};
 use crate::error::{AppError, AppResult};
@@ -32,13 +33,16 @@ fn default_limit() -> i64 {
 async fn list(
     State(state): State<AppState>,
     Query(params): Query<ListParams>,
-) -> AppResult<Json<Vec<Manga>>> {
+) -> AppResult<Json<PagedResponse<Manga>>> {
+    let limit = params.limit.clamp(1, 200);
+    let offset = params.offset.max(0);
     let q = repo::manga::ListQuery {
         search: params.search.filter(|s| !s.trim().is_empty()),
-        limit: params.limit.clamp(1, 200),
-        offset: params.offset.max(0),
+        limit,
+        offset,
     };
-    Ok(Json(repo::manga::list(&state.db, &q).await?))
+    let items = repo::manga::list(&state.db, &q).await?;
+    Ok(Json(PagedResponse::new(items, limit, offset)))
 }
 
 async fn get_one(