feat: add friendly error messages, MailDebugRecorder, clear-error endpoint, debug_logging column

Agent-Logs-Url: https://github.com/christianlouis/InboxConverge/sessions/5d2918de-630e-4a66-8355-1b036c620b1c

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

backend/alembic/versions/0002_add_debug_logging.py

@@ -0,0 +1,37 @@
+"""Add debug_logging column to mail_accounts
+
+Revision ID: 0002
+Revises: 0001
+Create Date: 2026-05-03
+
+Adds a boolean ``debug_logging`` column to ``mail_accounts``.
+
+When True, the next processing run will record a detailed connection trace
+(timings, phase-by-phase events, message UIDs/sizes) and persist it as a
+``ProcessingLog`` row with ``level='DEBUG'``.  The column auto-resets to
+False after 5 completed runs in a 24-hour window to prevent it being left
+on indefinitely.
+
+The column defaults to False so all existing rows are unaffected.
+Using IF NOT EXISTS makes the migration idempotent against fresh installs
+where create_all() already created the column.
+"""
+
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision = "0002"
+down_revision = "0001"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.execute(
+        "ALTER TABLE mail_accounts "
+        "ADD COLUMN IF NOT EXISTS debug_logging BOOLEAN NOT NULL DEFAULT FALSE"
+    )
+
+
+def downgrade() -> None:
+    op.execute("ALTER TABLE mail_accounts DROP COLUMN IF EXISTS debug_logging")