fix: "No files found." check produces false positives when AA returns results alongside that string (#1067)

## Problem

Searching Anna's Archive with multiple format filters (e.g. `epub`,
`mobi`, `pdf`, …) combined with a language filter (`lang=de`) and a
specific title produces a response that **contains both a full results
table and a `\"No files found.\"` span** in a separate section of the
page.

The previous code performed a naïve substring check on the raw HTML
before parsing:

```python
if "No files found." in html:
    logger.info("No books found for query: %s", query)
    return []
```

When Anna's Archive renders a page that has, e.g., results for one
content type but no results for another sub-filter, it emits a `<span
class="font-bold">No files found.</span>` in the empty sub-section while
the main results table is fully populated (650+ entries in the tested
case). The early return discards all those results, making Shelfmark
appear unable to find anything on Anna's Archive.

Reported upstream as: https://github.com/calibrain/shelfmark/issues/1042

## Root cause

The string `"No files found."` appears in multiple places on an AA
search results page. It is not a reliable signal that the entire query
returned zero results — only that *some* filtered sub-section is empty.

## Fix

Parse the HTML into BeautifulSoup first and look for the results
`<table>`. Only if no table is present does it make sense to fall back
to the string check:

```python
soup = BeautifulSoup(_html_response_text(html), "html.parser")
tbody = soup.find("table")

if tbody is None:
    if "No files found." in html:
        logger.info("No books found for query: %s", query)
        return []
    logger.warning("No results table found for query: %s", query)
    msg = "No books found. Please try another query."
    raise RuntimeError(msg)
```

This preserves both existing behaviours:
- Genuine empty results (no table + "No files found." present) → return
`[]`
- Unexpected response structure (no table, no "No files found." either)
→ raise `RuntimeError`

And fixes the false-positive case (table present + "No files found." in
another section) → proceed normally and parse the table.

## Testing

Verified on a self-hosted Shelfmark v1.3.0 instance against
`annas-archive.gl`:

- Query: `"Reise zum Mittelpunkt der Erde"` + `lang=de` + all supported
ebook formats
- Before fix: `search_books()` returned `[]` immediately
- After fix: `search_books()` returned 6 matching German epub/mobi
results

The fix is a pure restructuring — no logic is added or removed, only the
order of operations changes.

shelfmark/release_sources/direct_download.py

@@ -397,14 +397,13 @@ def search_books(query: str, filters: SearchFilters) -> list[BrowseRecord]:
         msg = "Unable to reach download source. Network restricted or mirrors are blocked."
         raise SearchUnavailableError(msg)
 
-    if "No files found." in html:
-        logger.info("No books found for query: %s", query)
-        return []
-
     soup = BeautifulSoup(_html_response_text(html), "html.parser")
     tbody = soup.find("table")
 
     if tbody is None:
+        if "No files found." in html:
+            logger.info("No books found for query: %s", query)
+            return []
         logger.warning("No results table found for query: %s", query)
         msg = "No books found. Please try another query."
         raise RuntimeError(msg)