feat: subdomain routing por tenant — refac-xxx.nexusautoparts.com

- Nginx wildcard config: *.nexusautoparts.com routes to POS app with X-Tenant-Subdomain header
- middleware_tenant.py: resolves subdomain -> tenant_id via nexus_master.tenants.subdomain
- auth_bp: login and employee list endpoints accept tenant from subdomain, URL param, or body
- login.html: auto-detects tenant from subdomain, shows business name, falls back to ?tenant=ID
- tenant_manager: generates subdomain slug from business name on provision_tenant()
- Migration v1.2: adds subdomain column + unique index to tenants table
- setup-nginx.sh: one-command install script for the nginx config

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pos/migrations/v1.2_subdomain.sql

@@ -0,0 +1,7 @@
+-- v1.2: Add subdomain column to tenants table (nexus_master)
+-- This migration runs against the MASTER database, not tenant databases.
+
+ALTER TABLE tenants ADD COLUMN IF NOT EXISTS subdomain VARCHAR(100) UNIQUE;
+CREATE INDEX IF NOT EXISTS idx_tenants_subdomain ON tenants(subdomain);
+
+COMMENT ON COLUMN tenants.subdomain IS 'URL subdomain for tenant POS access, e.g. refac-lopez -> refac-lopez.nexusautoparts.com';