fix(billing): importer review fixes — surface failures, validate, dedupe

Resolves findings from the post-build review:
- C1: a partial import was indistinguishable from success. action_run_import
  now logs failed rows at ERROR (survives nexa's log_level=warn) and the
  wizard shows red/amber banners with failed/skipped counts.
- H3: an unrecognized billing_cycle silently fell back to monthly (wrong
  plan AND price). Now raised per-row -> failed[], never silently mis-billed.
- M5: a NULL plan price silently became a $0 line. Prices now preserve
  NULL-vs-0.0; a missing price for the subscription's cycle is failed[].
- H2: post-connect query/schema errors now become a clean UserError, not a
  raw SQL traceback (matches the connection-error path).
- M4: per-row failures now record the exception type and log a traceback.
- MED#3: charge plan_id set explicitly False so re-runs re-assert the
  shadow-safe NULL even if it was changed between runs.
- HIGH-edge: re-run only rewrites x_fc_* on existing subs; partner_id/plan_id/
  line are set at creation only (never rewrite immutable fields).
- account_link: partner email match is now case-insensitive (=ilike) to avoid
  duplicate partners against a differently-cased pre-existing partner.

Shadow-safety invariant unchanged and re-confirmed. 52/52 green on odoo-trial.

fusion_centralize_billing/tests/test_importer.py

@@ -211,6 +211,29 @@ class TestImporterErrorIsolation(TransactionCase):
         self.assertTrue(summary['failed'], "the bad row must be recorded in failed[]")
         self.assertTrue(any(f['kind'] == 'user' for f in summary['failed']))
 
+    def test_unknown_billing_cycle_is_failed_not_silently_monthly(self):
+        data = _fixture()
+        data['subscriptions'][0]['billing_cycle'] = 'annual'  # not monthly/yearly
+        summary = self.Wizard._import_rows(data)
+        self.assertFalse(self.env['sale.order'].search(
+            [('x_fc_nexacloud_subscription_id', '=', 's-1')]),
+            "an unrecognized billing_cycle must NOT silently create a monthly sub")
+        self.assertTrue(any(f['kind'] == 'subscription' and f['id'] == 's-1'
+                            for f in summary['failed']))
+
+    def test_missing_price_for_cycle_is_failed_not_zero(self):
+        data = _fixture()
+        data['plans'][0]['price_yearly'] = None   # s-2 is yearly -> no price for it
+        summary = self.Wizard._import_rows(data)
+        # the yearly sub fails (no silent $0 line); the monthly one still imports
+        self.assertFalse(self.env['sale.order'].search(
+            [('x_fc_nexacloud_subscription_id', '=', 's-2')]),
+            "a missing price for the cycle must NOT silently create a $0 line")
+        self.assertTrue(self.env['sale.order'].search(
+            [('x_fc_nexacloud_subscription_id', '=', 's-1')]))
+        self.assertTrue(any(f['kind'] == 'subscription' and f['id'] == 's-2'
+                            for f in summary['failed']))
+
 
 @tagged('post_install', '-at_install')
 class TestImporterReadGuard(TransactionCase):