feat: track a rolling "since last tag" translator list

Reworks docs/changelog/translators.json into two lists maintained by
scripts/translators.sh:

- mappings: a forever-growing Crowdin-username/id -> npub registry. --seed
  appends new contributors with a blank npub and never deletes or overwrites
  existing entries.
- sinceLastTag: a rolling snapshot of who has translated since the last v* tag,
  fully refreshed on every --seed run.

The contribution window now defaults to the most recent v* tag instead of a
fixed two months (falling back to two months ago when no tag is reachable). The
CI seed job fetches tags (fetch-depth: 0) so it can resolve that window.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FWQWVdWLAwBUBX2gJ55y6b

.github/workflows/crowdin.yml

@@ -43,6 +43,10 @@ jobs:
     steps:
       - name: Checkout code
         uses: actions/checkout@v6
+        with:
+          # Need tags so the script can resolve the last v* release tag for the
+          # "since last tag" window.
+          fetch-depth: 0
 
       - name: Seed translator placeholders from Crowdin
         run: bash scripts/translators.sh --seed