add a arch federation overview

docs/unsure-where-to-put/federation-architecture.md

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+# Federation Architecture Principles
+
+While implementing federation in forgejo we introduced some conncepts from DomainDrivenDesign:
+
+1. **Aggregate**: Aggregates are clusters of objects (entities or values) which are handled atomic when it comes to persistence.
+2. **Validation**: Every object should express it's own validity, whenever someone is interested in
+   1. we collect as many invalidity information as possible in one shoot - so we return a list of validation issues if there are some.
+   2. Objects entering the lifetime are checked for validity on the borders (after loaded from and before stored to DB, after being newly created (New* functions) or after loaded via web / REST).
+
+Objects in forgefed package reflect Objects from ap or f3 lib but add some Forgejo specific enhancements like more specific validation.
+
+
+```mermaid
+classDiagram
+  namespace activitypub {
+    class Activity {
+      ID ID
+      Type ActivityVocabularyType // Like
+      Actor Item
+      Object Item
+    }
+    class Actor {
+      ID
+      Type ActivityVocabularyType // Person
+      Name NaturalLanguageValues
+      PreferredUsername NaturalLanguageValues
+      Inbox Item
+      Outbox Item
+      PublicKey PublicKey
+    }
+  }
+
+  namespace forgfed {
+    class ForgePerson {
+        Validate() []string
+    }
+    class ForgeLike {
+      Actor PersonID
+      Validate() []string
+    }
+    class ActorID {
+      ID               string
+      Schema           string
+      Path             string
+      Host             string
+      Port             string
+      UnvalidatedInput string
+      Validate() []string
+    }
+    class PersonID {
+      AsLoginName() string // "ID-Host"
+      AsWebfinger() string // "@ID@Host"
+      Validate() []string
+    }
+    class RepositoryID {
+      Validate() []string
+    }
+    class FederationHost {
+      <<Aggregate Root>>
+      ID int64
+      HostFqdn string
+      Validate() []string 
+    }
+
+    class NodeInfo {
+      Source string
+      Validate() []string
+    }
+  }
+
+  Actor <|-- ForgePerson
+  Activity <|-- ForgeLike
+  
+  ActorID <|-- PersonID
+  ActorID <|-- RepositoryID
+  ForgeLike *-- PersonID: Actor
+  ForgePerson -- PersonID: links to
+  FederationHost *-- NodeInfo
+
+  namespace user {
+    class User {
+      <<Aggregate Root>>
+      ID        int64
+      LowerName string
+      Name      string
+      Email     string
+      Passwd    string
+      LoginName   string
+      Type        UserType
+      IsActive bool
+      IsAdmin bool
+      Validate() []string
+    }
+
+    class FederatedUser {
+      ID         int64
+      UserID     int64
+      ExternalID   string
+      FederationHost int64
+      Validate() []string
+    }
+  }
+
+  User *-- FederatedUser: FederatedUser.UserID
+  PersonID -- FederatedUser : mapped by PersonID.ID == FederatedUser.externalID & FederationHost.ID
+  PersonID -- FederationHost : mapped by PersonID.Host == FederationHost.HostFqdn
+  FederatedUser -- FederationHost 
+```