feat: add migration to anonymize existing fall_ids

Replaces Nachname-based fall_ids with KVNR or random 6-char suffix
for all existing cases in the database.

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

backend/alembic/versions/006_anonymize_fall_ids.py

@@ -0,0 +1,42 @@
+"""Anonymize fall_ids: replace Nachname with KVNR or random suffix.
+
+Revision ID: 006_anonymize_fall_ids
+Revises: 005_add_disclosure_requests
+"""
+
+import random
+import string
+
+from alembic import op
+import sqlalchemy as sa
+
+revision = "006_anonymize_fall_ids"
+down_revision = "005_add_disclosure_requests"
+branch_labels = None
+depends_on = None
+
+
+def _random_suffix(length=6):
+    charset = string.ascii_uppercase + string.digits
+    return "".join(random.choices(charset, k=length))
+
+
+def upgrade():
+    conn = op.get_bind()
+    cases = conn.execute(
+        sa.text("SELECT id, fall_id, kvnr, jahr, kw, fallgruppe FROM cases")
+    ).fetchall()
+
+    for case in cases:
+        case_id, old_fall_id, kvnr, jahr, kw, fallgruppe = case
+        suffix = kvnr if kvnr else _random_suffix()
+        new_fall_id = f"{jahr}-{kw:02d}-{fallgruppe}-{suffix}"
+        conn.execute(
+            sa.text("UPDATE cases SET fall_id = :new_id WHERE id = :case_id"),
+            {"new_id": new_fall_id, "case_id": case_id},
+        )
+
+
+def downgrade():
+    # Cannot restore original Nachname-based fall_ids — data is lost
+    pass