[feature] Add fields introduced in Mastodon 4.4.0 to /api/v2/instance response (#4240)

# Description

## Checklist

- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [ ] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
- [x] I/we have not leveraged AI to create the proposed changes.
- [x] I/we have performed a self-review of added code.
- [x] I/we have written code that is legible and maintainable by others.
- [ ] I/we have commented the added code, particularly in hard-to-understand areas.
- [x] I/we have made any necessary changes to documentation.
- [ ] I/we have added tests that cover new code.
- [ ] I/we have run tests and they pass locally with the changes.
- [x] I/we have run `go fmt ./...` and `golangci-lint run`.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4240
Co-authored-by: nicole mikołajczyk <git@mkljczk.pl>
Co-committed-by: nicole mikołajczyk <git@mkljczk.pl>

docs/api/swagger.yaml

@@ -2108,6 +2108,20 @@ definitions:
                 example: <p>Registrations are currently closed on example.org because of spam bots!</p>
                 type: string
                 x-go-name: Message
+            min_age:
+                description: |-
+                    A minimum age required to register, if configured.
+                    Currently not implemented: will always be null.
+                format: int64
+                type: integer
+                x-go-name: MinAge
+            reason_required:
+                description: |-
+                    Whether registrations require the user to provide a reason for joining.
+                    Only applicable when ApprovalRequired is true.
+                example: true
+                type: boolean
+                x-go-name: ReasonRequired
         title: Information about registering for this instance.
         type: object
         x-go-name: InstanceV2Registrations
@@ -2178,11 +2192,26 @@ definitions:
         x-go-package: code.superseriousbusiness.org/gotosocial/internal/api/model
     instanceV2URLs:
         properties:
+            about:
+                description: Address of the server about page.
+                example: https://example.org/about
+                type: string
+                x-go-name: About
+            privacy_policy:
+                description: Address of the server privacy policy, if any.
+                example: https://example.org/about#rules
+                type: string
+                x-go-name: PrivacyPolicy
             streaming:
                 description: Websockets address for status and notification streaming.
                 example: wss://example.org
                 type: string
                 x-go-name: Streaming
+            terms_of_service:
+                description: Address of the server current terms of service, if any.
+                example: https://example.org/about#privacy_policy
+                type: string
+                x-go-name: TermsOfService
         title: InstanceV2URLs models instance-relevant URLs for client application consumption.
         type: object
         x-go-name: InstanceV2URLs

internal/api/model/instancev2.go

@@ -150,6 +150,15 @@ type InstanceV2URLs struct {
 	// Websockets address for status and notification streaming.
 	// example: wss://example.org
 	Streaming string `json:"streaming"`
+	// Address of the server about page.
+	// example: https://example.org/about
+	About string `json:"about"`
+	// Address of the server privacy policy, if any.
+	// example: https://example.org/about#rules
+	PrivacyPolicy *string `json:"privacy_policy"`
+	// Address of the server current terms of service, if any.
+	// example: https://example.org/about#privacy_policy
+	TermsOfService *string `json:"terms_of_service"`
 }
 
 // Hints related to translation.
@@ -199,6 +208,13 @@ type InstanceV2Registrations struct {
 	// Value will be null if no message is set.
 	// example: <p>Registrations are currently closed on example.org because of spam bots!</p>
 	Message *string `json:"message"`
+	// A minimum age required to register, if configured.
+	// Currently not implemented: will always be null.
+	MinAge *int `json:"min_age"`
+	// Whether registrations require the user to provide a reason for joining.
+	// Only applicable when ApprovalRequired is true.
+	// example: true
+	ReasonRequired bool `json:"reason_required"`
 }
 
 // Hints related to contacting a representative of the instance.

internal/typeutils/internaltofrontend.go

@@ -1889,8 +1889,12 @@ func (c *Converter) InstanceToAPIV2Instance(ctx context.Context, i *gtsmodel.Ins
 
 	instance.Thumbnail = thumbnail
 
+	termsOfService := config.GetProtocol() + "://" + i.Domain + "/about#rules"
+
 	// configuration
 	instance.Configuration.URLs.Streaming = "wss://" + i.Domain
+	instance.Configuration.URLs.About = config.GetProtocol() + "://" + i.Domain + "/about"
+	instance.Configuration.URLs.TermsOfService = &termsOfService
 	instance.Configuration.Statuses.MaxCharacters = config.GetStatusesMaxChars()
 	instance.Configuration.Statuses.MaxMediaAttachments = config.GetStatusesMediaMaxFiles()
 	instance.Configuration.Statuses.CharactersReservedPerURL = instanceStatusesCharactersReservedPerURL
@@ -1936,6 +1940,8 @@ func (c *Converter) InstanceToAPIV2Instance(ctx context.Context, i *gtsmodel.Ins
 	instance.Registrations.Enabled = config.GetAccountsRegistrationOpen()
 	instance.Registrations.ApprovalRequired = true // always required
 	instance.Registrations.Message = nil           // todo: not implemented
+	instance.Registrations.MinAge = nil            // not implemented
+	instance.Registrations.ReasonRequired = config.GetAccountsReasonRequired()
 
 	// contact
 	instance.Contact.Email = i.ContactEmail

internal/typeutils/internaltofrontend_test.go

@@ -2521,7 +2521,10 @@ func (suite *InternalToFrontendTestSuite) TestInstanceV2ToFrontend() {
   ],
   "configuration": {
     "urls": {
-      "streaming": "wss://localhost:8080"
+      "streaming": "wss://localhost:8080",
+      "about": "http://localhost:8080/about",
+      "privacy_policy": null,
+      "terms_of_service": "http://localhost:8080/about#rules"
     },
     "accounts": {
       "allow_custom_css": true,
@@ -2587,7 +2590,9 @@ func (suite *InternalToFrontendTestSuite) TestInstanceV2ToFrontend() {
   "registrations": {
     "enabled": true,
     "approval_required": true,
-    "message": null
+    "message": null,
+    "min_age": null,
+    "reason_required": true
   },
   "contact": {
     "email": "admin@example.org",