test: unit tests for the SEO classes

Pure tests of the classes package (no database): the resolver's value
chains with every layer present and absent, robots per directive, the
staging lock and drafts; the redirect graph (chains, flattened and
back-and-forth graphs, loops by exact, pattern and regex probes, the
10-hop cap); URL normalisation across every address rule; the robots
grammar over every directive combination; Google's robots.txt matcher
examples; templates and unknown placeholders; CSV export and import;
owner resolution; and every kind of the issue report.

go.mod: gorm.io/driver/postgres becomes a direct requirement (the tests'
dry-run database).

go.mod

@@ -11,6 +11,7 @@ require (
 	github.com/testcontainers/testcontainers-go v0.44.0
 	github.com/testcontainers/testcontainers-go/modules/postgres v0.44.0
 	gocloud.dev v0.46.0
+	gorm.io/driver/postgres v1.6.3
 	gorm.io/gorm v1.31.2
 )
 
@@ -178,7 +179,6 @@ require (
 	google.golang.org/genproto/googleapis/rpc v0.0.0-20260316180232-0b37fe3546d5 // indirect
 	google.golang.org/grpc v1.79.3 // indirect
 	google.golang.org/protobuf v1.36.11 // indirect
-	gorm.io/driver/postgres v1.6.3 // indirect
 )
 
 replace git.golem15.com/golem15/summercms => ../summercms.go