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/models/account_link.py

@@ -46,7 +46,9 @@ class FusionBillingAccountLink(models.Model):
             return existing
         partner = self.env['res.partner']
         if email:
-            partner = partner.search([('email', '=', email)], limit=1)
+            # case-insensitive so a pre-existing partner with a differently-cased email
+            # (created via the web UI or another sync) is reused, not duplicated.
+            partner = partner.search([('email', '=ilike', email)], limit=1)
         if not partner:
             partner = partner.create({'name': name or external_id, 'email': email, **(extra or {})})
         return self.create({