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/auth_token.go

@@ -0,0 +1,37 @@
+package models
+
+import (
+	"context"
+	"time"
+)
+
+// AuthToken represents an authentication token for a user session.
+type AuthToken struct {
+	Base
+
+	Token      string     `json:"-"`
+	UserID     string     `json:"userId"`
+	CreatedAt  time.Time  `json:"createdAt"`
+	ExpiresAt  *time.Time `json:"expiresAt,omitempty"`
+	LastUsedAt *time.Time `json:"lastUsedAt,omitempty"`
+}
+
+// User returns the user who owns this token.
+func (t *AuthToken) User(ctx context.Context) (*User, error) {
+	u := &User{}
+	u.SetDB(t.db)
+
+	err := t.GetDB().QueryRowContext(ctx, `
+		SELECT id, nick, password_hash, created_at, updated_at, last_seen_at
+		FROM users WHERE id = ?`,
+		t.UserID,
+	).Scan(
+		&u.ID, &u.Nick, &u.PasswordHash,
+		&u.CreatedAt, &u.UpdatedAt, &u.LastSeenAt,
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	return u, nil
+}