feat: initial DataMigration extension for case data backup/restore

Full case data migration tool with 3 console commands:
- data-migration-export: exports all case-related entities (27 types) + files
- data-migration-import: restores backup to target DB with dry-run support
- data-migration-status: shows entity counts from live DB or backup

Supports preserve-ids, skip-duplicates, batch processing, and attachment
file copying. Imports with hooks disabled to prevent side effects.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

manifest.json

@@ -0,0 +1,13 @@
+{
+    "name": "DataMigration",
+    "version": "1.0.0",
+    "acceptableVersions": [
+        ">=8.0.0"
+    ],
+    "php": [
+        ">=8.1"
+    ],
+    "releaseDate": "2026-03-28",
+    "author": "klear",
+    "description": "Case data migration: export/import all case-related data between EspoCRM instances"
+}