feat: simplify registry number format to B-2026-00001

New format: single-letter prefix + year + 5-digit sequence.
No direction code (IN/OUT) in the number — shown via arrow icon.
Sequence is shared across directions within the same company+year.

Changes:
- REGISTRY_COMPANY_PREFIX: BTG→B, USW→U, SDT→S, GRP→G
- OLD_COMPANY_PREFIX map for backward compat with existing entries
- allocateSequenceNumber: searches both old and new format entries
  to find the actual max sequence (backward compat)
- recalculateSequence: same dual-format search
- parseRegistryNumber: supports 3 formats (current, v1, legacy)
- isNewFormat: updated regex for B-2026-00001
- CompactNumber: already used single-letter badges, just updated comment
- debug-sequences endpoint: updated for new format
- Notification test data: updated to new format
- RegistrySequence.type: now "SEQ" (shared) instead of "IN"/"OUT"

After deploy: POST /api/registratura/debug-sequences to clean up
old counters, then recreate test entries.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

prisma/schema.prisma

@@ -87,9 +87,9 @@ model GisUat {
 
 model RegistrySequence {
   id        String   @id @default(uuid())
-  company   String   // BTG, SDT, USW, GRP
+  company   String   // B, U, S, G (single-letter prefix)
   year      Int
-  type      String   // IN, OUT, INT
+  type      String   // SEQ (shared across directions)
   lastSeq   Int      @default(0)
   createdAt DateTime @default(now())
   updatedAt DateTime @updatedAt