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/model.go

@@ -1,9 +1,11 @@
 // Package models defines the data models used by the chat application.
+// All model structs embed Base, which provides database access for
+// relation-fetching methods directly on model instances.
 package models
 
 import "database/sql"
 
-// DB is the interface that models use to query relations.
+// DB is the interface that models use to query the database.
 // This avoids a circular import with the db package.
 type DB interface {
 	GetDB() *sql.DB
@@ -18,3 +20,8 @@ type Base struct {
 func (b *Base) SetDB(d DB) {
 	b.db = d
 }
+
+// GetDB returns the database interface for use in model methods.
+func (b *Base) GetDB() *sql.DB {
+	return b.db.GetDB()
+}