Document SQLite DSN defaults explicitly

The sample environment file now shows the same SQLite DSN shape used by the application default, including shared cache, foreign keys, WAL journaling, normal synchronous mode, and a busy timeout. The config test now pins the generated SQLite DSN exactly so future changes do not silently drop those pragmas.

Constraint: SQLite remains the default and OWL_DB_DSN can stay empty to derive from OWL_DB_PATH
Confidence: high
Scope-risk: narrow
Directive: Keep the SQLite example DSN and config.sqliteDSN helper in sync
Tested: cd backend && go test ./internal/config
Tested: git diff --check

Author: Owl Builder

.env.example

@@ -12,7 +12,7 @@ OWL_DB_PATH=/app/data/data.db
 #
 # SQLite (default): leave OWL_DB_DSN empty to use OWL_DB_PATH, or set an explicit DSN.
 # OWL_DB_TYPE=sqlite
-# OWL_DB_DSN=file:/app/data/data.db?_fk=1
+# OWL_DB_DSN=file:/app/data/data.db?cache=shared&_pragma=foreign_keys(1)&_pragma=journal_mode(WAL)&_pragma=synchronous(NORMAL)&_pragma=busy_timeout(10000)
 #
 # PostgreSQL example:
 # OWL_DB_TYPE=postgres

backend/internal/config/config_test.go

@@ -46,8 +46,9 @@ func TestLoadDatabaseDefaultsToSQLiteDSN(t *testing.T) {
 	if cfg.DatabaseDriver != "sqlite3" {
 		t.Fatalf("expected sqlite3 driver, got %q", cfg.DatabaseDriver)
 	}
-	if cfg.DatabaseDSN == "" || cfg.DatabaseDSN == dbPath {
-		t.Fatalf("expected generated sqlite DSN, got %q", cfg.DatabaseDSN)
+	expectedDSN := "file:" + dbPath + "?cache=shared&_pragma=foreign_keys(1)&_pragma=journal_mode(WAL)&_pragma=synchronous(NORMAL)&_pragma=busy_timeout(10000)"
+	if cfg.DatabaseDSN != expectedDSN {
+		t.Fatalf("expected generated sqlite DSN %q, got %q", expectedDSN, cfg.DatabaseDSN)
 	}
 }