Add image uploads in chat messages (Gitea issue #10)

Images live on the app server's local disk (uploads/), served through an
authenticated, room-membership-gated endpoint since rooms can be private.
Uploads are streamed with a byte-count cap, validated as genuine decodable
images with Pillow (not just a spoofed Content-Type), and downscaled to
2000px on the longer side (except GIF, to preserve animation).

Backend: MessageImage model + nullable Message.content/image_id with a
content-or-image CheckConstraint, upload/serve endpoints in rooms.py, WS
message envelope gains image_id, push notification body says "sent an
image" for image-only messages.

Frontend: Composer gets an attach button with upload progress and a
thumbnail chip; MessageList renders images inline with a click-to-zoom
ImageLightbox.

backend/app/models/message.py

@@ -1,7 +1,7 @@
 import uuid
 from datetime import datetime
 
-from sqlalchemy import DateTime, ForeignKey, Text, func
+from sqlalchemy import CheckConstraint, DateTime, ForeignKey, Text, func
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from app.models.base import Base
@@ -9,11 +9,21 @@ from app.models.base import Base
 
 class Message(Base):
     __tablename__ = "messages"
+    __table_args__ = (
+        CheckConstraint(
+            "content IS NOT NULL OR image_id IS NOT NULL",
+            name="messages_content_or_image_required",
+        ),
+    )
 
     id: Mapped[uuid.UUID] = mapped_column(primary_key=True, default=uuid.uuid4)
     room_id: Mapped[uuid.UUID] = mapped_column(ForeignKey("rooms.id"), index=True, nullable=False)
     user_id: Mapped[uuid.UUID] = mapped_column(ForeignKey("users.id"), nullable=False)
-    content: Mapped[str] = mapped_column(Text, nullable=False)
+    # Nullable since Phase "image uploads": a message can be an image with
+    # no caption. The CheckConstraint above still requires at least one of
+    # content/image_id.
+    content: Mapped[str | None] = mapped_column(Text)
+    image_id: Mapped[uuid.UUID | None] = mapped_column(ForeignKey("message_images.id"))
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), server_default=func.now(), index=True, nullable=False
     )
@@ -21,3 +31,4 @@ class Message(Base):
     deleted_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
 
     user = relationship("User")
+    image = relationship("MessageImage")