Add admin-configurable upload size limits

The 8MB image/file/avatar cap is now a site setting (UploadSettings,
single-row table like SmtpSettings) editable from the Admin Settings tab,
instead of a hardcoded constant. All three upload endpoints read the live
value and interpolate it into their 413 messages. A new GET
/api/uploads/limit endpoint (open to any authenticated user, unlike the
admin-only settings endpoints) lets the composer reject an oversized file
client-side before it ever hits the network, though the server still
enforces the same cap independently.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -13,6 +13,7 @@ from app.models.push_subscription import PushSubscription
 from app.models.room import Room
 from app.models.site_invite import SiteInvite
 from app.models.smtp_settings import SmtpSettings
+from app.models.upload_settings import UploadSettings
 from app.models.user import User
 from app.models.webhook_incoming import WebhookIncoming
 
@@ -30,6 +31,7 @@ __all__ = [
     "PasswordReset",
     "SiteInvite",
     "SmtpSettings",
+    "UploadSettings",
     "PushSubscription",
     "AdminAuditLog",
     "ApiToken",

backend/app/models/upload_settings.py

@@ -0,0 +1,21 @@
+import uuid
+from datetime import datetime
+
+from sqlalchemy import BigInteger, DateTime, func
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.models.base import Base
+
+
+class UploadSettings(Base):
+    """A single-row table (enforced in the service layer, not the schema --
+    same convention as SmtpSettings) holding the site-wide max upload size
+    for images and file attachments, set through the Admin UI."""
+
+    __tablename__ = "upload_settings"
+
+    id: Mapped[uuid.UUID] = mapped_column(primary_key=True, default=uuid.uuid4)
+    max_upload_bytes: Mapped[int] = mapped_column(BigInteger, nullable=False)
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now(), onupdate=func.now(), nullable=False
+    )