feat(periods): undo klarmarkera so an externally closed year can be reopened (#1978)

markPeriodClosedExternally ("klarmarkera") closes and locks an imported
year without a closing entry, and nothing could reverse it: unlockPeriod
refuses closed periods and the SIE replace flow refuses closed or locked
years. An owner who klarmarkerade five imported years and then found the
prior-year SIE file was wrong had no way back (Forsslund Systems,
2026-08-27).

reopenExternallyClosedPeriod reverses the mark while the closed state still
comes from klarmarkera (closed_externally set, no closing entry), clears the
lock, writes the audit_log row, and emits period.unlocked. New route
POST /api/bookkeeping/fiscal-periods/[id]/reopen-external with envelope codes
PERIOD_REOPEN_NOT_CLOSED / PERIOD_REOPEN_NOT_EXTERNAL; "Öppna igen" action
and "Avslutat i tidigare program" chip in Settings > Bookkeeping > Fiscal
years; unlock and SIE replace refusals now point at that path.

Co-authored-by: Jakob Wennberg <311770904+jakobwennberg-oss@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

lib/core/bookkeeping/__tests__/period-service.test.ts

@@ -34,6 +34,7 @@ import {
   unlockPeriod,
   closePeriod,
   markPeriodClosedExternally,
+  reopenExternallyClosedPeriod,
   createNextPeriod,
   findNextPeriod,
   resolvePeriodStatusForDate,
@@ -766,6 +767,84 @@ describe('markPeriodClosedExternally', () => {
   })
 })
 
+describe('reopenExternallyClosedPeriod', () => {
+  it('reopens a klarmarkerad period, clears the lock, and writes the audit row', async () => {
+    const period = makeFiscalPeriod({
+      id: 'fp-1',
+      is_closed: true,
+      closed_at: '2026-08-27T09:20:13Z',
+      closed_externally: true,
+      locked_at: '2026-08-27T09:20:13Z',
+      closing_entry_id: null,
+    })
+    const reopened = {
+      ...period,
+      is_closed: false,
+      closed_at: null,
+      closed_externally: false,
+      locked_at: null,
+    }
+    results = [
+      { data: period, error: null },    // fetch
+      { data: reopened, error: null },  // update
+      { data: null, error: null },      // audit_log insert
+    ]
+
+    const handler = vi.fn()
+    eventBus.on('period.unlocked', handler)
+
+    const supabase = makeClient()
+    const result = await reopenExternallyClosedPeriod(supabase as never, 'company-1', 'user-1', 'fp-1')
+
+    expect(result.is_closed).toBe(false)
+    expect(result.closed_externally).toBe(false)
+    expect(result.locked_at).toBeNull()
+    expect(handler).toHaveBeenCalledOnce()
+    // Three table touches: fetch, update, audit_log.
+    expect(supabase.from).toHaveBeenCalledWith('audit_log')
+  })
+
+  it('rejects an open period', async () => {
+    const period = makeFiscalPeriod({ id: 'fp-1', is_closed: false, closed_externally: false })
+    results = [{ data: period, error: null }]
+
+    const supabase = makeClient()
+    await expect(
+      reopenExternallyClosedPeriod(supabase as never, 'company-1', 'user-1', 'fp-1')
+    ).rejects.toThrow('not closed')
+  })
+
+  it('rejects a period closed by a year-end run (closing entry, not klarmarkera)', async () => {
+    const period = makeFiscalPeriod({
+      id: 'fp-1',
+      is_closed: true,
+      closed_externally: false,
+      closing_entry_id: 'ce-1',
+    })
+    results = [{ data: period, error: null }]
+
+    const supabase = makeClient()
+    await expect(
+      reopenExternallyClosedPeriod(supabase as never, 'company-1', 'user-1', 'fp-1')
+    ).rejects.toThrow('year-end run')
+  })
+
+  it('rejects a klarmarkerad period that later got its own closing entry', async () => {
+    const period = makeFiscalPeriod({
+      id: 'fp-1',
+      is_closed: true,
+      closed_externally: true,
+      closing_entry_id: 'ce-1',
+    })
+    results = [{ data: period, error: null }]
+
+    const supabase = makeClient()
+    await expect(
+      reopenExternallyClosedPeriod(supabase as never, 'company-1', 'user-1', 'fp-1')
+    ).rejects.toThrow('year-end run')
+  })
+})
+
 describe('unlockPeriod', () => {
   it('clears locked_at and emits period.unlocked', async () => {
     const period = makeFiscalPeriod({