chore: prune unused deps, undocumented env vars, and stale recipes

[eetu]

A second pass over accumulated drift, after the formatting one in #116. Five
commits, each independent — nothing here changes runtime behaviour.

chore: drop dependencies nothing uses

Twelve dependency declarations that no source file references:

crate	unused
backend	async-trait, base64, tokio-util
press	axum-extra, thiserror, tower, tower-http
shared	serde_json
shelf	tokio-util
frontend	classnames

press carried thiserror without defining a single error type — Error
appears nowhere under press/src.

Three more sat in [workspace.dependencies] that no member ever picked up:
hyper and hyper-util, which were never referenced by anything even before
this change, and base64, orphaned by the backend removal.

Found by grep, then confirmed the only way that counts — removing them and
rebuilding. Only async-trait actually leaves the dependency graph; the rest
stay as transitive deps of other crates, so this is mostly manifest honesty
rather than a build-time win.

docs: document three env vars, and the e2e crate

SCRIBE_COVERS_DIR, STATIC_DIR and SHELF_COVERS_DIR are read at boot but
were in no .env.example — the only way to find them was reading config.rs.
The other ~30 vars are all documented, so these three were oversights rather
than a convention.

The covers pair earns a real comment: backend and shelf must point at the
same directory, because shelf only serves cover art backend has already
cached and never writes any of its own. Their defaults disagree — backend
falls back to a relative covers, shelf to the container path /data/covers
— so a shared-host dev setup that leaves both unset gets silently missing
artwork rather than an error. Both files now say so and cross-reference.

README also claimed four workspace members (there are five) and omitted the
e2e crate from the layout entirely.

chore(justfile): make check cover the shim, and stop implying test runs e2e

Two recipes promised more than they delivered:

• check called itself "lint + format + typecheck everything" but skipped
  the Python shim, though the hook and CI both run ruff over it.
• test called itself "run all tests", but cargo test --workspace walks
  straight past the e2e suite — it's #[ignore]d, so no filter short of
  --ignored reaches it. Nothing local ran it, so checking that the stack
  still boots meant hand-copying the command out of ci.yaml.

Added just e2e with CI's exact invocation and reworded test. Longer prose
moved into the recipe bodies, because just --list shows only the last
comment line above a recipe — a multi-line doc comment renders as a dangling
fragment.

chore(frontend): regenerate routeTree.gen.ts

The committed generated output predated the pinned @tanstack/router-plugin,
so any yarn dev/yarn build left the working tree dirty. Ordering-only:
same four routes, emitted ascending instead of reversed.

ci: correct the stale crate list in the cache comment

The rust-cache comment named three workspace members and "all three crates";
there are five. shelf and e2e joined later and the comment never caught up.
Comment only.

Verification

• cargo build --workspace --all-targets — clean
• cargo clippy --workspace --all-targets -- -D warnings — clean
• cargo test --workspace — 43 passed
• just e2e — stack boots, 1 test passes in 3.3 s
• yarn validate and yarn build — clean
• just --list renders every description correctly
• just check passes end to end on this branch, shim ruff step included

Note

Separately, and not a repo problem: my local frontend/node_modules had
drifted to prettier 3.9.5 against a lockfile pinning 3.9.6, which made
yarn format — and therefore the pre-commit hook on any frontend commit —
fail with command not found: prettier. A plain yarn install fixed it, and
the lockfile needed no change.