Add complete database schema and ORM models

Schema (002_tables.sql):
- users: accounts with nick, password hash, timestamps
- auth_tokens: per-device tokens with expiry, linked to users
- channels: chat rooms with topic and mode flags
- channel_members: membership with per-user modes (+o, +v)
- messages: channel/DM history with structured JSON meta
- message_queue: per-user pending delivery queue
- sessions: server-held session state with idle timeout
- server_links: federation peer configuration

Models (internal/models/):
- All models embed Base for database access
- Relation methods on models: User.Channels(), User.QueuedMessages(),
  Channel.Members(), Channel.RecentMessages(), ChannelMember.User(),
  ChannelMember.Channel(), AuthToken.User(), Session.User()
- IDs are UUIDs (TEXT), not auto-increment integers
- JSON tags use camelCase per lint rules

All tables verified: migrations apply cleanly, 0 lint issues.

internal/models/server_link.go

@@ -0,0 +1,18 @@
+package models
+
+import (
+	"time"
+)
+
+// ServerLink represents a federation peer server configuration.
+type ServerLink struct {
+	Base
+
+	ID            string     `json:"id"`
+	Name          string     `json:"name"`
+	URL           string     `json:"url"`
+	SharedKeyHash string     `json:"-"`
+	IsActive      bool       `json:"isActive"`
+	CreatedAt     time.Time  `json:"createdAt"`
+	LastSeenAt    *time.Time `json:"lastSeenAt,omitempty"`
+}