Derive every key from the signing key in one place (closes #315)

seal.DeriveKeys derives the session hash and block keys, the encrypted URL
key and the CSRF key from the signing key, with their salts declared beside
it, and returns them as one seal.Keys. internal/providers calls it once at
startup and provides the value to the fx graph. The session manager, the
encrypted URL generator and the CSRF protection each take their own key.
session.NewManager, encurl.NewGenerator, newCSRFProtect and handlers.New
return no error, as nothing in them can fail now. Each key is derived as
before, with seal.DeriveKey and the same salt. The signing key itself stays
the HMAC key of signed URLs and the login password. README.md says under
signing_key what it protects.

Model: opus-5-5

internal/handlers/csrf.go

@@ -9,15 +9,10 @@ import (
 	"sneak.berlin/go/pixa/internal/seal"
 )
 
-// csrfKeySalt provides domain separation for the CSRF authentication key,
-// derived from the signing key so tokens survive restarts without extra
-// configuration and never reuse the session or encrypted-URL key material.
-const csrfKeySalt = "pixa-csrf-v1"
-
 // newCSRFProtect builds the CSRF-protection middleware for the
 // state-mutating HTML form routes. The token lives in its own cookie,
 // independent of the session cookie, so it also protects the login POST
-// where no session exists yet (login CSRF).
+// where no session exists yet (login CSRF). key signs that cookie.
 //
 // When plaintext is true (local HTTP development), requests are marked
 // plaintext so the library neither demands an https Referer nor sets a
@@ -25,13 +20,8 @@ const csrfKeySalt = "pixa-csrf-v1"
 // service runs behind a TLS-terminating proxy, so plaintext is false and
 // the library enforces its https Referer origin check.
 func newCSRFProtect(
-	signingKey string, plaintext bool,
-) (func(http.Handler) http.Handler, error) {
-	key, err := seal.DeriveKey([]byte(signingKey), csrfKeySalt)
-	if err != nil {
-		return nil, err
-	}
-
+	key [seal.KeySize]byte, plaintext bool,
+) func(http.Handler) http.Handler {
 	protect := csrf.Protect(
 		key[:],
 		csrf.Path("/"),
@@ -40,7 +30,7 @@ func newCSRFProtect(
 	)
 
 	if !plaintext {
-		return protect, nil
+		return protect
 	}
 
 	return func(next http.Handler) http.Handler {
@@ -49,7 +39,7 @@ func newCSRFProtect(
 		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 			protected.ServeHTTP(w, csrf.PlaintextHTTPRequest(r))
 		})
-	}, nil
+	}
 }
 
 // CSRF returns the CSRF-protection middleware for the login and generator