feat(postcards): add notes field to postcards with edit, detail, and truncated list display

app/models.py

@@ -83,6 +83,8 @@ class Postcard(Base):
     # received
     sender_name = Column(String(64))
     receive_time = Column(DateTime)
+    # misc
+    notes = Column(Text, nullable=False, default="")
     # images
     showcase_hidden = Column(default=False, nullable=False)
     image_front = Column(String(256))