Add job brackets for grouping multi-part or repeated jobs

- JobBracket model: name, customer, notes
- Migration 0006: job_brackets table + bracket_id FK on print_jobs
- /brackets: list with per-bracket progress bar
- /brackets/{id}: detail with jobs table + Add Job(s) modal (quantity field
  creates N identical jobs in one shot, each with its own UUID and log entry)
- /jobs Add Job modal: optional bracket selector
- Sidebar: Brackets nav item added

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/models/__init__.py

@@ -1,5 +1,6 @@
 from app.models.base import Base
 from app.models.filament import Filament
+from app.models.job_bracket import JobBracket
 from app.models.job_log import JobLog
 from app.models.pricing import PricingConfig
 from app.models.print_job import PrintJob
@@ -8,4 +9,4 @@ from app.models.printer_log import PrinterLog
 from app.models.printer_type import PrinterType
 from app.models.todo import Todo
 
-__all__ = ["Base", "Printer", "PrinterLog", "PrinterType", "Filament", "JobLog", "PrintJob", "Todo", "PricingConfig"]
+__all__ = ["Base", "Printer", "PrinterLog", "PrinterType", "Filament", "JobBracket", "JobLog", "PrintJob", "Todo", "PricingConfig"]