feat: enable Markdown (.md) file upload in KB Accelerator

Moved md from Phase 2 extensions to allowed formats, added extraction
handler (reuses txt handler), and updated plan_limits defaults to
include md for all plans.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/alembic/versions/056_add_md_to_kb_allowed_formats.py

@@ -0,0 +1,44 @@
+"""Add md to kb_allowed_formats defaults
+
+Revision ID: 056
+Revises: 055
+Create Date: 2026-03-12
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "056"
+down_revision = "055"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # Update server default for new rows
+    op.alter_column(
+        "plan_limits",
+        "kb_allowed_formats",
+        server_default=sa.text("'[\"txt\",\"paste\",\"md\"]'::jsonb"),
+    )
+    # Add "md" to existing rows that have the old default ["txt","paste"]
+    op.execute(
+        """
+        UPDATE plan_limits
+        SET kb_allowed_formats = kb_allowed_formats || '["md"]'::jsonb
+        WHERE NOT kb_allowed_formats @> '"md"'::jsonb
+        """
+    )
+
+
+def downgrade() -> None:
+    op.alter_column(
+        "plan_limits",
+        "kb_allowed_formats",
+        server_default=sa.text("'[\"txt\",\"paste\"]'::jsonb"),
+    )
+    op.execute(
+        """
+        UPDATE plan_limits
+        SET kb_allowed_formats = kb_allowed_formats - 'md'
+        """
+    )