Spotnet import, header backfill, staging and counter grouping, faster search

Spotnet (CONCEPT D29):
- Index free.pt fully and import spots with their descriptions, categories
  and NZBs; a spot renames its release, since Spotnet names are more
  trustworthy than names derived from subjects.
- Find passwords in spot descriptions too, and tell "needs a password we
  have" from "needs a password we don't have".

Content inspection:
- List the contents of 7z (.7z.001) archives, SFV sets, nicovideo uploads
  and other archive-like files; mark yEncBin .bin containers as encrypted.
- Take real names from yEnc headers for randomly named files, and merge
  uploads that were split across posts.

Storage (D31, D32):
- Store segment lists zstd-compressed behind the FileStore accessor.
- group:backfill indexes older headers at low priority and pauses when
  the NVMe or HDD part of the database gets too large.

Grouping (D35, D36):
- Lone header segments wait in staged_segments until a second segment of
  their file arrives, which drops the per-article junk binaries.
- Counter grouping joins random-subject uploads by poster, [i/M] counter,
  part count and day.

Search and maintenance (D33, D34):
- A trigram-indexed search_text column, smaller indexes, estimated stats.
- Passes that clean up nonsense names; weekly index maintenance; GC budgets
  per kind.
- Web UI: grey out stale results while a search runs and cancel superseded
  requests. Count only up to 1,000 matches for the page, which made filter
  clicks take seconds (Newznab keeps the 10,000 cap).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

public/index.html

@@ -39,7 +39,7 @@
         <div class="filters" role="group" aria-label="Filters">
           <button class="chip" type="button" data-filter="readable" aria-pressed="false">Readable names</button>
           <button class="chip" type="button" data-filter="complete" aria-pressed="false">Complete</button>
-          <button class="chip" type="button" data-filter="hide_encrypted" aria-pressed="false">Hide passworded</button>
+          <button class="chip" type="button" data-filter="hide_encrypted" aria-pressed="false" title="Hide releases that need a password we don't know">Hide password needed</button>
           <button class="chip" type="button" data-filter="par2_only" aria-pressed="false" title="Also show releases that are only par2 files whose data never arrived">Show par2-only</button>
           <label class="select select-small">
             <span class="visually-hidden">Recognised by</span>