feat(zettle): sync paid purchases into webshop_orders (#2445)

Community PR #2416 by @olofpinzke, adopted and finished by maintainers (rebased so every commit is signed).

Why the problem occurred: no Zettle integration; POS sales only reached the books as bank descriptors while Woo/Shopify already had order underlag via webshop_orders. The contributor's version also failed at the database (platform CHECKs listed only woocommerce/shopify), which the mocked unit tests never saw.
What was simplified: reused the Orders/book/invoice path instead of a new inbox; Finance API payouts/fees deferred. Sales the one-account, revenue-per-rate model cannot book (split tender, gift cards, tips) import unbookable with a "bokför manuellt" title instead of guessing accounts. Reset parity uses the rename-and-wrap pattern instead of re-issuing the reset body.
Why this solution: per-purchase rows give the radunderlag BFL verifikat need and the bulk-book path exists; daily kassarapport aggregation and Finance API fees/payouts are the follow-up (DECISIONS.md). Skeptic-refuted paths fixed before merge: concurrent refresh-token rotation (sync claim), cron offset paging (candidate snapshot), platform CHECKs, writer-role gate, migration-reset parity, white-label return origin re-validated at callback, VAT net from product rows.

Not live until ZETTLE_CLIENT_ID / ZETTLE_CLIENT_SECRET / ZETTLE_CREDENTIALS_ENCRYPTION_KEY are set on Vercel and a Zettle developer app is registered with the callback redirect URI.

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WtYqzKPoTSRHskYYdf7MwB

lib/extensions/__tests__/sectors.test.ts

@@ -48,8 +48,8 @@ describe('sectors registry', () => {
     expect(SECTORS.length).toBe(1)
   })
 
-  it('should have 18 total extensions', () => {
-    expect(getAllExtensions().length).toBe(18)
+  it('should have 19 total extensions', () => {
+    expect(getAllExtensions().length).toBe(19)
   })
 
   it('should have unique slugs within each sector', () => {
@@ -94,7 +94,7 @@ describe('sectors registry', () => {
 
   it('getExtensionsBySector returns extensions for a sector', () => {
     const extensions = getExtensionsBySector('general')
-    expect(extensions.length).toBe(18)
+    expect(extensions.length).toBe(19)
   })
 
   it('all extensions have required fields', () => {