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/tests/health.rs

@@ -7,7 +7,7 @@ use tower::ServiceExt;
 #[sqlx::test(migrations = "./migrations")]
 async fn health_returns_ok(pool: PgPool) {
     let h = common::harness(pool);
-    let resp = h.app.oneshot(common::get("/api/health")).await.unwrap();
+    let resp = h.app.oneshot(common::get("/api/v1/health")).await.unwrap();
     assert_eq!(resp.status(), StatusCode::OK);
     let body = common::body_json(resp).await;
     assert_eq!(body["status"], "ok");