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/message_queue.go

@@ -0,0 +1,15 @@
+package models
+
+import (
+	"time"
+)
+
+// MessageQueueEntry represents a pending message delivery for a user.
+type MessageQueueEntry struct {
+	Base
+
+	ID        int64     `json:"id"`
+	UserID    string    `json:"userId"`
+	MessageID string    `json:"messageId"`
+	QueuedAt  time.Time `json:"queuedAt"`
+}