Add printer lifecycle fields and printer log

- Printer: bought_at, last_maintenance_at, decommissioned_at (all Date, nullable)
- PrinterLog: per-printer timestamped text entries (CASCADE delete)
- Migration 0003: adds columns + printer_logs table
- /printers/{id}: detail page showing lifecycle info + scrollable log
- POST /printers/{id}/log: add a log entry via modal form
- Printer names in list are now links to the detail page
- Add Printer modal includes optional purchase date

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

app/models/printer_log.py

@@ -0,0 +1,17 @@
+from datetime import datetime
+
+from sqlalchemy import DateTime, ForeignKey, Text, func
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.models.base import Base
+
+
+class PrinterLog(Base):
+    __tablename__ = "printer_logs"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    printer_id: Mapped[int] = mapped_column(ForeignKey("printers.id"))
+    message: Mapped[str] = mapped_column(Text)
+    created_at: Mapped[datetime] = mapped_column(DateTime, server_default=func.now())
+
+    printer: Mapped["Printer"] = relationship(back_populates="logs")