feat(server)!: require a logged-in account for every channel request

Drop unauthenticated guests from channels: the allow_guests setting, the
/guests endpoints and the guest seat table are gone, and requests without an
account session get 401 account_required. Share links are unaffected.
Contract version is now 8.0.0.

contracts/api.schemas.json

@@ -1,6 +1,6 @@
 {
   "$schema": "https://json-schema.org/draft/2020-12/schema",
-  "x-backend-contract-sha256": "54ea3f00fa672e0241efbb9fc8e7ad9e8fc68415ec124e08f70657c2d57d9ff9",
+  "x-backend-contract-sha256": "655e918f3443073c270bdeb9ae4e69d79dacf83baa90dea1a0cdcfe71d2b8bd5",
   "$defs": {
     "AccountChannelBody": {
       "type": "object",
@@ -362,9 +362,6 @@
     "ChannelSettings": {
       "type": "object",
       "properties": {
-        "allow_guests": {
-          "type": "boolean"
-        },
         "delete_policy": {
           "$ref": "#/$defs/DeletePolicy"
         },
@@ -401,7 +398,6 @@
         }
       },
       "required": [
-        "allow_guests",
         "delete_policy",
         "file_max_downloads",
         "join_policy",
@@ -415,16 +411,6 @@
     "ChannelSettingsPatch": {
       "type": "object",
       "properties": {
-        "allow_guests": {
-          "anyOf": [
-            {
-              "type": "boolean"
-            },
-            {
-              "type": "null"
-            }
-          ]
-        },
         "delete_policy": {
           "anyOf": [
             {
@@ -841,21 +827,6 @@
         "oldest_seq"
       ]
     },
-    "GuestInfo": {
-      "type": "object",
-      "properties": {
-        "endpoint_hash": {
-          "type": "string"
-        },
-        "joined_at": {
-          "type": "integer"
-        }
-      },
-      "required": [
-        "endpoint_hash",
-        "joined_at"
-      ]
-    },
     "InvitationBody": {
       "type": "object",
       "properties": {