feat(imap): add IMAP_READONLY_MODE feature flag to safeguard shared mailboxes

When enabled, IMAP processing will fetch and process attachments but
will NOT modify the mailbox state (no starring, labeling, deleting,
or flag changes). This allows preprod instances to safely share a
Gmail inbox with production without interfering with production
email processing.

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/config.py

@@ -221,6 +221,15 @@ class Settings(BaseSettings):
 
     # Feature flags
     allow_file_delete: bool = True  # Default to allowing file deletion from database
+    imap_readonly_mode: bool = Field(
+        default=False,
+        description=(
+            "When enabled, IMAP processing will fetch and process attachments but will NOT modify "
+            "the mailbox state (no starring, labeling, deleting, or flag changes). "
+            "Use this for pre-production instances that share a mailbox with production to prevent "
+            "preprod from interfering with production email processing."
+        ),
+    )
 
     # Batch processing settings
     processall_throttle_threshold: int = Field(