Run SQLite in WAL mode with a busy timeout the driver reads (closes #123)

The default database URL asked for WAL mode with `_journal_mode` and
`_busy_timeout`, which the SQLite driver does not read, so the database
ran in rollback-journal mode. The default now uses the driver's
`_pragma=journal_mode(WAL)` and `_pragma=busy_timeout(5000)` parameters,
in one constant that both the config default and the database fallback
use. The database no longer sets its own busy timeout after opening, so
the URL's value applies. A new test opens a file database with those
parameters through db.New and checks the journal mode and busy timeout.
README.md's database text now matches, and an in-memory test database
drops the same ignored parameter.

Model: opus-5-5
Co-authored-by: clawbot <sneak+clawbot@sneak.cloud>

internal/config/config.go

@@ -21,6 +21,12 @@ const defaultMOTD = ` _ __   ___  ___  (_)_ __ ___
 Welcome to NeoIRC — IRC semantics over HTTP.
 Type /help for available commands.`
 
+// DefaultDBURL is the database URL used when DBURL is not set. Its
+// _pragma parameters, which the SQLite driver runs on every connection
+// it opens, set WAL mode and a 5000 ms busy timeout.
+const DefaultDBURL = "file:///var/lib/neoirc/state.db" +
+	"?_pragma=journal_mode(WAL)&_pragma=busy_timeout(5000)"
+
 // Params defines the dependencies for creating a Config.
 type Params struct {
 	fx.In
@@ -71,7 +77,7 @@ func New(
 	viper.SetDefault("DEBUG", "false")
 	viper.SetDefault("MAINTENANCE_MODE", "false")
 	viper.SetDefault("PORT", "8080")
-	viper.SetDefault("DBURL", "file:///var/lib/neoirc/state.db?_journal_mode=WAL")
+	viper.SetDefault("DBURL", DefaultDBURL)
 	viper.SetDefault("SENTRY_DSN", "")
 	viper.SetDefault("METRICS_USERNAME", "")
 	viper.SetDefault("METRICS_PASSWORD", "")