feat(pos): add real thermal ticket printing via ESC/POS (#21)

- Add thermal_printer.py service generating raw ESC/POS bytes for 58mm/80mm printers
- Add /pos/api/sales/<id>/print endpoint (escpos_raw or browser mode)
- Add printer.js with WebUSB and Web Serial support for direct browser-to-printer
- Add thermal print button in ticket modal with connect/print workflow

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

pos/blueprints/pos_bp.py

@@ -1698,3 +1698,96 @@ def push_test():
     if ok:
         return jsonify({'message': 'Test notification sent'})
     return jsonify({'error': 'No subscription found or push failed'}), 400
+
+
+# ─── Thermal Printing ──────────────────────────────
+
+@pos_bp.route('/sales/<int:sale_id>/print', methods=['POST'])
+@require_auth('pos.sell')
+def print_ticket(sale_id):
+    """Generate a printable ticket for a sale.
+
+    Body (optional): {printer_type: 'escpos_raw' | 'browser', width: 58 | 80}
+      - escpos_raw: returns raw ESC/POS bytes (application/octet-stream)
+      - browser: returns printable HTML fragment (text/html)
+    """
+    from flask import Response
+    from services.thermal_printer import generate_ticket
+
+    body = request.get_json(silent=True) or {}
+    printer_type = body.get('printer_type', 'escpos_raw')
+    width = int(body.get('width', 80))
+
+    conn = get_tenant_conn(g.tenant_id)
+    cur = conn.cursor()
+
+    # Fetch sale
+    cur.execute("""
+        SELECT s.*, e.name as employee_name, c.name as customer_name
+        FROM sales s
+        LEFT JOIN employees e ON s.employee_id = e.id
+        LEFT JOIN customers c ON s.customer_id = c.id
+        WHERE s.id = %s
+    """, (sale_id,))
+    row = cur.fetchone()
+    if not row:
+        cur.close(); conn.close()
+        return jsonify({'error': 'Sale not found'}), 404
+
+    cols = [desc[0] for desc in cur.description]
+    sale = dict(zip(cols, row))
+    for k in ('subtotal', 'discount_total', 'tax_total', 'total', 'amount_paid', 'change_given'):
+        if sale.get(k) is not None:
+            sale[k] = float(sale[k])
+
+    # Fetch items
+    cur.execute("""
+        SELECT name, quantity, unit_price, subtotal
+        FROM sale_items WHERE sale_id = %s ORDER BY id
+    """, (sale_id,))
+    items = []
+    for r in cur.fetchall():
+        items.append({
+            'name': r[0], 'quantity': r[1],
+            'unit_price': float(r[2]) if r[2] else 0,
+            'subtotal': float(r[3]) if r[3] else 0,
+        })
+
+    # Fetch business info from config
+    business_info = {'name': 'NEXUS AUTOPARTS', 'rfc': '', 'address': ''}
+    try:
+        cur.execute("SELECT key, value FROM config WHERE key IN ('business_name','rfc','address')")
+        for rw in cur.fetchall():
+            if rw[0] == 'business_name':
+                business_info['name'] = rw[1]
+            else:
+                business_info[rw[0]] = rw[1]
+    except Exception:
+        pass
+
+    cur.close()
+    conn.close()
+
+    sale_data = {
+        'folio': f'V-{sale["id"]}',
+        'date': str(sale.get('created_at', '')),
+        'employee': sale.get('employee_name', ''),
+        'customer': sale.get('customer_name', ''),
+        'items': items,
+        'subtotal': sale.get('subtotal', 0),
+        'discount_total': sale.get('discount_total', 0),
+        'tax_total': sale.get('tax_total', 0),
+        'total': sale.get('total', 0),
+        'payment_method': sale.get('payment_method', 'efectivo'),
+        'amount_paid': sale.get('amount_paid'),
+        'change_given': sale.get('change_given'),
+    }
+
+    if printer_type == 'browser':
+        # Return the sale data as JSON for browser-side rendering
+        return jsonify(sale_data)
+
+    # Default: ESC/POS raw bytes
+    raw = generate_ticket(sale_data, business_info, width=width)
+    return Response(raw, mimetype='application/octet-stream',
+                    headers={'Content-Disposition': f'attachment; filename=ticket_{sale_id}.bin'})