fix(accounting): search on a filter Paperless actually honours

[PhilippTheServer]

Closes #55 · found while seeding the new Paperless service with data

GET /v1/admin/accounting/documents?query=… returned every document, whatever the term.

The router mapped query onto a Paperless filter named text. Paperless has no such
filter, and ignores unknown parameters rather than rejecting them — so the request
succeeded, returned 200, and handed back the whole collection.

Measured against 183 seeded documents:

GET /api/documents/?query=Barolo           ->  25   (Paperless, correct)
GET /api/documents/?title_content=Barolo   ->  25   (Paperless, correct)
GET /api/documents/?text=Barolo            -> 183   (unknown filter, ignored)

GET /v1/admin/accounting/documents?query=Barolo -> 183

An operator looking for one invoice gets a full result list that reads as a search matching
a lot. Nothing errors, nothing warns.

The fix

build_document_filters moves the mapping out of the endpoint so it can be tested without a
running Paperless, and PAPERLESS_FILTERS pins every outgoing name against the set
Paperless recognises — because a wrong name here never fails loudly, it just silently widens
the results.

The existing test asserted the buggy value ({"text": "invoice"}), so it is updated
rather than deleted. It was locking the defect in.

Also here

/v1/admin/analytics/storefront gains a limit, matching the /products endpoint beside
it. Its product_interest and top_paths were fixed at ten rows with no way to ask for
more — which surfaced when a shop with real traffic pushed an integration test's own fixture
SKU out of the top ten.

Verification

$ uv run pytest tests/test_accounting_filters_unit.py -q
9 passed

$ uv run pytest -q
1067 passed, 7 skipped in 18.26s

$ ruff check src/app tests/
All checks passed!

The tests catch the bug. Restoring text:

E  assert 'text' not in {'page': 1, 'page_size': 50, 'text': 'Barolo'}
E  AssertionError: assert 'text' == 'query'
FAILED test_full_text_search_uses_a_filter_paperless_honours