feat: check formatting, linting and tests in CI

Every push runs ruff, prettier, pytest and svelte-check; a tagged release runs the
blocking half again before it publishes an image. eslint and svelte-check report
without failing, since 87 and 30 findings predate the workflow.

frontend/AGENTS.md

@@ -163,9 +163,12 @@ Observed in the current tree — don't mistake these for intentional patterns to
   current; regenerate it again after any backend API change, with
   `pnpm exec openapi-typescript http://localhost:8000/schema/openapi.json -o src/lib/schema/openapi/schema.d.ts`
   against a backend running **your** branch — a stale server silently writes a stale file.
-- `pnpm lint` does not pass either — 131 pre-existing ESLint errors, 35 of them
-  `svelte/no-navigation-without-resolve` on plain `href`s, plus ~59 files Prettier would rewrite
-  (mostly vendored shadcn components). Check the files you touched, not the whole tree.
+- **Prettier is clean and CI blocks on it** — run `pnpm format` before finishing. Two things it
+  must not touch are in `.prettierignore`: the vendored foliate-js, and
+  `src/lib/schema/openapi/schema.d.ts`, which `openapi-typescript` regenerates in its own style.
+- `pnpm exec eslint .` reports **87 pre-existing errors as of 2026-08-17**, all in `src/`. CI runs
+  it non-blocking (`continue-on-error`) until that reaches zero. `static/pdfjs/` is ignored
+  alongside `src/lib/vendor/` — it is vendored too, and linting it produced 1717 further errors.
 - `src/routes/api/[...path]/+server.ts` — all four handlers are annotated `RequestHandler` while the
   import of that type is commented out at line 4. It also buffers whole **responses** with
   `arrayBuffer()` and forwards no `Range` header, so book downloads are not streamed. **Requests**