chore: format the tree and clear ruff's findings

Applies ruff format, ruff check --fix and prettier, so CI can gate on all three.
The surviving unused imports were all re-exports in __init__.py, now covered by a
per-file ignore; the pdf.js viewer and the generated openapi types join the
vendored code that eslint and prettier already skip.

frontend/.prettierignore

@@ -8,9 +8,14 @@ bun.lockb
 # Ignore artifacts:
 build
 coverage
+.pytest_cache
 
 # Miscellaneous
 /static/
 
 # Vendored third-party source, copied verbatim by scripts/vendor-foliate.sh
 /src/lib/vendor/
+
+# Generated by openapi-typescript, which has its own formatting. Reformatting it here would
+# make every regeneration a several-thousand-line diff.
+/src/lib/schema/openapi/schema.d.ts