Move the package into src/

Thirty-seven entries in the root, most of them .go files. The assets had to
come along: //go:embed cannot reach outside its own directory, so
templates/, static/ and migrations/ live beside the code that embeds them,
and testdata/ beside the test that reads it. storage/ stays put — runtime
data, not source.

go build now needs -o. Without it the output would be named after the
package directory and collide with src/ itself.

Makefile

@@ -16,8 +16,10 @@ vet:
 test:
 	go test ./...
 
+# -o is required, not stylistic: the package lives in ./src, and without it `go build` would try to
+# write a binary named "src" over the directory.
 build:
-	go build -o levyraati26-go .
+	go build -o levyraati26-go ./src
 
 # Templates and migrations are embedded, so seeing a change means rebuilding.
 run: build