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/print_job.py

@@ -24,6 +24,7 @@ class PrintJob(Base):
     job_uuid: Mapped[str | None] = mapped_column(String(36), unique=True, default=lambda: str(_uuid.uuid4()))
     printer_id: Mapped[int | None] = mapped_column(ForeignKey("printers.id"))
     filament_id: Mapped[int | None] = mapped_column(ForeignKey("filaments.id"))
+    bracket_id: Mapped[int | None] = mapped_column(ForeignKey("job_brackets.id"))
     name: Mapped[str] = mapped_column(String(200))
     customer: Mapped[str | None] = mapped_column(String(200))
     file_name: Mapped[str | None] = mapped_column(String(255))
@@ -42,6 +43,7 @@ class PrintJob(Base):
 
     printer: Mapped["Printer"] = relationship(back_populates="jobs")
     filament: Mapped["Filament | None"] = relationship(back_populates="jobs")
+    bracket: Mapped["JobBracket | None"] = relationship(back_populates="jobs")
     logs: Mapped[list["JobLog"]] = relationship(
         back_populates="job", order_by="JobLog.created_at.desc()"
     )