set a (very large) alt text length limit

The motivation is to have *some* size limit on every string in post
records, to maximize interoperation. For example, we currently have a
CBOR library rejecting some records because of too-long strings.

We don't want to limit the ability of folks to be very descriptive in
alt text, specifically, so chose what seems to be a very large limit. If
this is not large enough, based on feedback, we can bump it even higher.
As context this is the largest string length limit in all of our
lexicons.

lexicons/app/bsky/embed/images.json

@@ -23,7 +23,11 @@
           "accept": ["image/*"],
           "maxSize": 1000000
         },
-        "alt": { "type": "string" },
+        "alt": {
+          "type": "string",
+          "maxGraphemes": 5000,
+          "maxLength": 50000
+        },
         "aspectRatio": { "type": "ref", "ref": "#aspectRatio" }
       }
     },
@@ -53,7 +57,11 @@
       "properties": {
         "thumb": { "type": "string" },
         "fullsize": { "type": "string" },
-        "alt": { "type": "string" },
+        "alt": {
+          "type": "string",
+          "maxGraphemes": 5000,
+          "maxLength": 50000
+        },
         "aspectRatio": { "type": "ref", "ref": "#aspectRatio" }
       }
     }