submissions(#23): foundation — kind-30083 schema, builder, service + tests

First slice of structured submissions (replacing the kind-4 encrypted DM forms).
Pure, hermetic core — no UI or network yet.

- nostrEventSchemas.js: SUBMISSION_KIND=30083 (+ NOTE_TYPES.SUBMISSION, BASE_SCHEMAS
  entry) and buildSubmissionEvent(). PUBLIC + addressable; data in tags: d (id),
  k (type), applicant (npub), p (admin), t=bitvid-submission; free-text in content.
- js/submissions/submissionService.js: SUBMISSION_TYPES + labels, parseSubmissionEvent,
  buildSubmission, generateSubmissionId. Pure — unit-testable with no relays/signer.
- constants.js: FEATURE_SUBMISSIONS (default OFF until the admin UI + forms ship).
- tests/submission-service.test.mjs: 8 scenarios (build tags/type/default, d-tag
  required, parse structure/reject/unknown-type→other, build→parse round-trip of
  every type, id generation).
- check-file-size.mjs: bump nostrEventSchemas grandfather 3250→3541 (adding a new
  event type to the schema registry is the prescribed pattern).
- docs/nostr-event-schemas.md: catalogue row.

Schema validation ✨, lint, build all pass; tests 8/8.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

scripts/check-file-size.mjs

@@ -32,7 +32,7 @@ const GRANDFATHERED = {
   "js/channelProfile.js": 5529,
   "js/nostr/client.js": 4356,
   "js/ui/components/VideoCard.js": 3129,
-  "js/nostrEventSchemas.js": 3250,
+  "js/nostrEventSchemas.js": 3541,
   "js/ui/loginModalController.js": 2552,
   "js/historyView.js": 2613,
   "js/services/moderationService.js": 2512,