Remove REST API, keep GraphQL as sole API

- Add missing GraphQL mutations: retryJob, updateAsset, deleteAsset
- Add UpdateAssetRequest and DeleteResult to schema source of truth
- Move Lambda callback endpoint to main.py (only REST endpoint)
- Remove REST routes, pydantic schemas, and deps
- Remove pydantic target from modelgen.json
- Update architecture diagrams and documentation

schema/modelgen.json

@@ -6,16 +6,6 @@
       "output": "mpr/media_assets/models.py",
       "include": ["dataclasses", "enums"]
     },
-    {
-      "target": "pydantic",
-      "output": "api/schema/",
-      "include": ["dataclasses", "enums"],
-      "name_map": {
-        "TranscodeJob": "Job",
-        "MediaAsset": "Asset",
-        "TranscodePreset": "Preset"
-      }
-    },
     {
       "target": "graphene",
       "output": "api/schema/graphql.py",