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. Handler tests and the relay tests' aliceWithHTTPClient
helper need a password only to log in, so they store one hashed at
bcrypt.MinCost. Tests of PASS itself keep the real cost.

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
+}