feat(ownership): build snapshots from XLSX with monthly history

- build-latest-ownership-snapshot.sh: discover with a wider limit and
  add --history <n> to import the n previous supported above-1% XLSX
  months (one per as-of date, oldest first) before the latest, so
  `idx ownership changes` works straight from a synced snapshot.
- publish-ownership-snapshot.sh: pass --history through (for the
  clan-private systemd timer via extraArgs).
- publish-ownership-snapshot.yml: `history` input (default 5).
- Docs: publish/self-hosted flow now describes the XLSX source,
  observed upload days (2nd-3rd of the month), and --history.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017a5Cv5Z1NzdYKgKnTiAF4E

.github/workflows/publish-ownership-snapshot.yml

@@ -7,6 +7,10 @@ on:
         description: Stable GitHub release tag to publish the snapshot assets to
         required: true
         default: ownership-snapshot-current
+      history:
+        description: Number of earlier monthly above-1% reports to include for `ownership changes`
+        required: false
+        default: "5"
 
 permissions:
   contents: write
@@ -36,7 +40,8 @@ jobs:
             --idx-bin ./target/debug/idx \
             --output-dir dist/ownership-snapshot \
             --repo "${{ github.repository }}" \
-            --release-tag "${{ inputs.release_tag }}"
+            --release-tag "${{ inputs.release_tag }}" \
+            --history "${{ inputs.history }}"
 
       - name: Upload workflow artifact copy
         uses: actions/upload-artifact@v4