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/services/upload_settings_service.py

@@ -0,0 +1,33 @@
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.models import UploadSettings
+from app.storage import DEFAULT_MAX_UPLOAD_BYTES
+
+
+async def get_upload_settings(db: AsyncSession) -> UploadSettings:
+    """Unlike SmtpSettings (where "no row yet" means "unconfigured, skip"),
+    an upload cap must always resolve to a usable value -- so this creates
+    the row with the default on first read instead of returning None."""
+    result = await db.execute(select(UploadSettings).limit(1))
+    settings_row = result.scalar_one_or_none()
+    if settings_row is None:
+        settings_row = UploadSettings(max_upload_bytes=DEFAULT_MAX_UPLOAD_BYTES)
+        db.add(settings_row)
+        await db.commit()
+        await db.refresh(settings_row)
+    return settings_row
+
+
+async def update_upload_settings(db: AsyncSession, *, max_upload_bytes: int) -> UploadSettings:
+    settings_row = await get_upload_settings(db)
+    settings_row.max_upload_bytes = max_upload_bytes
+    await db.commit()
+    await db.refresh(settings_row)
+    return settings_row
+
+
+def format_mb(num_bytes: int) -> str:
+    """Used in 413 error messages, which need to reflect the live
+    admin-configured cap rather than a hardcoded "8 MB"."""
+    return f"{num_bytes / (1024 * 1024):g} MB"