feat(server): split file storage into a separate file service with tickets, receipts and WebDAV backup

- new projects WebDrop.FileApi (file service), WebDrop.FileProtocol (shared tickets and receipts) and WebDrop.Sync (WebDAV)
- file content of any size goes to the file service; chat, avatars and space structure stay on the main backend
- tickets issued by the main backend, receipts by the file service, deletion queue and daily reconciliation
- daily database snapshot and file sync to external storage (OpenList/WebDAV) with a 30 day retention
- reorganize Core, Storage and Api into feature folders (namespaces unchanged)
- web client uploads file frames to the file service and reads them with tickets

contracts/api.schemas.json

@@ -1,6 +1,6 @@
 {
   "$schema": "https://json-schema.org/draft/2020-12/schema",
-  "x-backend-contract-sha256": "96cd0ffa3588a288af7bc720b2413cb56499dd862ac98d83c73ccc11a1147d25",
+  "x-backend-contract-sha256": "1b3b8d56bf4262d61d7c2814fcd788b09225ccc94120084673ea46de46c09593",
   "$defs": {
     "AccountChannelBody": {
       "type": "object",
@@ -844,6 +844,60 @@
         "error"
       ]
     },
+    "FileGrantBody": {
+      "type": "object",
+      "properties": {
+        "expires_at": {
+          "type": "integer"
+        },
+        "mode": {
+          "type": "string"
+        },
+        "ticket": {
+          "anyOf": [
+            {
+              "type": "string"
+            },
+            {
+              "type": "null"
+            }
+          ]
+        },
+        "url": {
+          "anyOf": [
+            {
+              "type": "string"
+            },
+            {
+              "type": "null"
+            }
+          ]
+        }
+      },
+      "required": [
+        "expires_at",
+        "mode"
+      ]
+    },
+    "FileGrantRequest": {
+      "type": "object",
+      "properties": {
+        "from": {
+          "type": "integer"
+        },
+        "item": {
+          "type": "string"
+        },
+        "op": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "from",
+        "item",
+        "op"
+      ]
+    },
     "FinishShareRequest": {
       "type": "object",
       "properties": {
@@ -1350,6 +1404,9 @@
         "defaults": {
           "$ref": "#/$defs/DefaultsBody"
         },
+        "files": {
+          "$ref": "#/$defs/FilesInfoBody"
+        },
         "limits": {
           "$ref": "#/$defs/LimitsBody"
         },
@@ -1365,6 +1422,7 @@
         "choices",
         "contract_version",
         "defaults",
+        "files",
         "limits",
         "protocol",
         "transports"
@@ -1414,6 +1472,27 @@
         "share"
       ]
     },
+    "FilesInfoBody": {
+      "type": "object",
+      "properties": {
+        "enabled": {
+          "type": "boolean"
+        },
+        "url": {
+          "anyOf": [
+            {
+              "type": "string"
+            },
+            {
+              "type": "null"
+            }
+          ]
+        }
+      },
+      "required": [
+        "enabled"
+      ]
+    },
     "LimitsBody": {
       "type": "object",
       "properties": {