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/views/import_wizard_views.xml

@@ -5,6 +5,13 @@
         <field name="model">fusion.billing.import.wizard</field>
         <field name="arch" type="xml">
             <form string="Import from NexaCloud">
+                <div class="alert alert-danger" role="alert" invisible="failed_count == 0">
+                    <strong>Import completed with errors: </strong>
+                    <field name="failed_count" class="oe_inline" readonly="1"/> row(s) failed — see Result below.
+                </div>
+                <div class="alert alert-warning" role="alert" invisible="skipped_count == 0">
+                    <field name="skipped_count" class="oe_inline" readonly="1"/> row(s) skipped (unresolved customer/plan) — see Result below.
+                </div>
                 <group>
                     <field name="dry_run"/>
                 </group>