Migrations: relax session sql_mode so the chain runs on strict MySQL 8

Found by test-deploying on a Windows + MySQL 8.0 VM: migration 7a01 seeds the
canonical relationship types with a raw INSERT that omits the NOT-NULL
createddate/modifieddate columns (the ORM supplies those via Python defaults at
runtime, but a raw migration INSERT does not). MySQL 5.x's lax default sql_mode
accepted it; strict MySQL 8 rejects it with 1364 "Field 'createddate' doesn't
have a default value", so a fresh `flask db upgrade` died at 7a01. Dev runs
MySQL 5.6, so this never surfaced locally.

migrations/env.py now sets the migration session sql_mode to
NO_ENGINE_SUBSTITUTION (dropping STRICT_TRANS_TABLES) for the migration run
only - the app's own runtime connections keep their mode. Makes the whole chain
portable across MySQL versions. Guarded for non-MySQL (sqlite tests).

68 migration/smoke tests pass; fresh upgrade to head verified on MySQL.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

migrations/env.py

@@ -139,6 +139,20 @@ def run_migrations_online():
         pass
 
     with connectable.connect() as connection:
+        # Relax the session sql_mode for the migration run. Some historical
+        # migrations seed reference rows with raw INSERTs that omit NOT-NULL
+        # timestamp columns (the ORM supplies those via Python defaults at
+        # runtime, but a raw migration INSERT does not). MySQL 5.x's lax default
+        # accepted that; strict MySQL 8 rejects it with 1364 "Field 'createddate'
+        # doesn't have a default value". Dropping STRICT_TRANS_TABLES for the
+        # migration session only (the app's own connections keep their mode)
+        # makes the chain portable across MySQL versions.
+        try:
+            connection.exec_driver_sql(
+                "SET SESSION sql_mode = 'NO_ENGINE_SUBSTITUTION'")
+        except Exception:
+            pass  # non-MySQL backends (e.g. sqlite in tests)
+
         context.configure(
             connection=connection,
             target_metadata=get_metadata(),