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>

frontend/src/lib/api/mangas.ts

@@ -1,4 +1,4 @@
-import { request, type Manga } from './client';
+import { request, type Manga, type Page } from './client';
 
 export type ListOptions = {
     search?: string;
@@ -6,17 +6,22 @@ export type ListOptions = {
     offset?: number;
 };
 
-export async function listMangas(opts: ListOptions = {}): Promise<Manga[]> {
+export type MangasPage = {
+    items: Manga[];
+    page: Page;
+};
+
+export async function listMangas(opts: ListOptions = {}): Promise<MangasPage> {
     const params = new URLSearchParams();
     if (opts.search) params.set('search', opts.search);
     if (opts.limit != null) params.set('limit', String(opts.limit));
     if (opts.offset != null) params.set('offset', String(opts.offset));
     const qs = params.toString();
-    return request<Manga[]>(`/mangas${qs ? `?${qs}` : ''}`);
+    return request<MangasPage>(`/v1/mangas${qs ? `?${qs}` : ''}`);
 }
 
 export async function getManga(id: string): Promise<Manga> {
-    return request<Manga>(`/mangas/${encodeURIComponent(id)}`);
+    return request<Manga>(`/v1/mangas/${encodeURIComponent(id)}`);
 }
 
 export type NewManga = {
@@ -26,11 +31,11 @@ export type NewManga = {
 };
 
 export async function createManga(input: NewManga): Promise<Manga> {
-    return request<Manga>('/mangas', {
+    return request<Manga>('/v1/mangas', {
         method: 'POST',
         headers: { 'content-type': 'application/json' },
         body: JSON.stringify(input)
     });
 }
 
-export type { Manga };
+export type { Manga, Page };