Move the test-only helpers out of the shipped db package (closes #248)

The server binary carried exported names that only tests call, including
raw SQL access and a way to weaken the password hash.

Delete testing.go, GetDB and SetBcryptCost from the shipped code, so
passwords always hash at bcrypt.DefaultCost. Move GetDB, GetOrCreateChannel,
JoinChannel and HasChannelInvite into export_test.go. Delete
GetClientHostInfo, whose remaining check GetLatestClientForSession covers.

Tests in other packages now build the database with db.New and run raw SQL
through dbtest.Exec.

Model: opus-5-5

internal/db/export_test.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"log/slog"
 	"testing"
+	"time"
 
 	"sneak.berlin/go/neoirc/internal/dbtest"
 )
@@ -67,3 +68,78 @@ func (database *Database) ExecForTest(
 
 	return nil
 }
+
+// GetDB returns the underlying sql.DB connection.
+func (database *Database) GetDB() *sql.DB {
+	return database.conn
+}
+
+// GetOrCreateChannel returns channel id, creating if needed.
+// Uses INSERT OR IGNORE to avoid TOCTOU races.
+func (database *Database) GetOrCreateChannel(
+	ctx context.Context,
+	name string,
+) (int64, error) {
+	now := time.Now()
+
+	_, err := database.conn.ExecContext(ctx,
+		`INSERT OR IGNORE INTO channels
+		 (name, created_at, updated_at)
+		 VALUES (?, ?, ?)`,
+		name, now, now)
+	if err != nil {
+		return 0, fmt.Errorf("create channel: %w", err)
+	}
+
+	var channelID int64
+
+	err = database.conn.QueryRowContext(
+		ctx,
+		"SELECT id FROM channels WHERE name = ?",
+		name,
+	).Scan(&channelID)
+	if err != nil {
+		return 0, fmt.Errorf("get channel: %w", err)
+	}
+
+	return channelID, nil
+}
+
+// JoinChannel adds a session to a channel.
+func (database *Database) JoinChannel(
+	ctx context.Context,
+	channelID, sessionID int64,
+) error {
+	_, err := database.conn.ExecContext(ctx,
+		`INSERT OR IGNORE INTO channel_members
+		 (channel_id, session_id, joined_at)
+		 VALUES (?, ?, ?)`,
+		channelID, sessionID, time.Now())
+	if err != nil {
+		return fmt.Errorf("join channel: %w", err)
+	}
+
+	return nil
+}
+
+// HasChannelInvite checks if a session has been invited
+// to a channel.
+func (database *Database) HasChannelInvite(
+	ctx context.Context,
+	channelID, sessionID int64,
+) (bool, error) {
+	var count int
+
+	err := database.conn.QueryRowContext(ctx,
+		`SELECT COUNT(*) FROM channel_invites
+		 WHERE channel_id = ? AND session_id = ?`,
+		channelID, sessionID,
+	).Scan(&count)
+	if err != nil {
+		return false, fmt.Errorf(
+			"check invite: %w", err,
+		)
+	}
+
+	return count > 0, nil
+}