Fix code review feedback items 1-6, 8-10

- Item 1: Extract GetUserByID/GetChannelByID lookup methods, use from relation methods
- Item 2: Initialize slices with literals so JSON gets [] not null
- Item 3: Populate CreatedAt/UpdatedAt with time.Now() on all Create methods
- Item 4: Wrap each migration's SQL + recording in a transaction
- Item 5: Check error from res.LastInsertId() in QueueMessage
- Item 6: Add DequeueMessages and AckMessages methods
- Item 8: Add GetUserByNick, GetUserByToken, DeleteAuthToken, UpdateUserLastSeen
- Item 9: Run PRAGMA foreign_keys = ON on every new connection
- Item 10: Builds clean, all tests pass

internal/models/model.go

@@ -3,7 +3,10 @@
 // relation-fetching methods directly on model instances.
 package models
 
-import "database/sql"
+import (
+	"context"
+	"database/sql"
+)
 
 // DB is the interface that models use to query the database.
 // This avoids a circular import with the db package.
@@ -11,6 +14,16 @@ type DB interface {
 	GetDB() *sql.DB
 }
 
+// UserLookup provides user lookup by ID without circular imports.
+type UserLookup interface {
+	GetUserByID(ctx context.Context, id string) (*User, error)
+}
+
+// ChannelLookup provides channel lookup by ID without circular imports.
+type ChannelLookup interface {
+	GetChannelByID(ctx context.Context, id string) (*Channel, error)
+}
+
 // Base is embedded in all model structs to provide database access.
 type Base struct {
 	db DB
@@ -25,3 +38,21 @@ func (b *Base) SetDB(d DB) {
 func (b *Base) GetDB() *sql.DB {
 	return b.db.GetDB()
 }
+
+// GetUserLookup returns the DB as a UserLookup if it implements the interface.
+func (b *Base) GetUserLookup() UserLookup {
+	if ul, ok := b.db.(UserLookup); ok {
+		return ul
+	}
+
+	return nil
+}
+
+// GetChannelLookup returns the DB as a ChannelLookup if it implements the interface.
+func (b *Base) GetChannelLookup() ChannelLookup {
+	if cl, ok := b.db.(ChannelLookup); ok {
+		return cl
+	}
+
+	return nil
+}