Add automatic instruction fetcher for official sets

Downloads official instruction PDFs from LEGO, located via the Brickset API, one set per interval (10 minutes by default) so neither API gets hammered. Official sets only; MOCs are excluded throughout.

- New admin page at /admin/instruction-fetch: queue builder, pacing controls, live countdown and per-set job history

- Database-backed queue (instruction_fetch_jobs) and last-run timestamp, so pacing survives restarts

- Background worker thread, plus 'flask instructions queue/run/status' for Task Scheduler or cron

- SHA-256 dedupe (instructions.sha256), PDF header check, size cap and streaming downloads via temp files

- brickset_api: getInstructions2 by set number and getInstructions by setID (the old code sent a set number to the setID method); getSets params now proper JSON; getKeyUsageStats added

- Schema top-ups applied automatically at boot: new table plus instructions.sha256/source

- Docker: fetcher env vars passed through compose, no worker thread in the entrypoint bootstrap

app/templates/admin/settings.html

@@ -8,6 +8,9 @@
         <h1><i class="bi bi-sliders"></i> Site Settings</h1>
     </div>
     <div class="col-auto">
+        <a href="{{ url_for('instruction_fetch.index') }}" class="btn btn-outline-warning me-2">
+            <i class="bi bi-cloud-arrow-down"></i> Instruction Fetcher
+        </a>
         <a href="{{ url_for('backup.index') }}" class="btn btn-outline-danger me-2">
             <i class="bi bi-archive"></i> Backup &amp; Restore
         </a>