feat(backend): per-Reader userscript credential with UI install and rotation (#24)

Each Reader's userscript credential is derived from TOKEN_KEY, their
Discord id and a token epoch (HMAC-SHA256, hex); only its SHA-256 sits in
readers.token_sha256, so install URLs survive restarts while a database
leak yields nothing but hashes. One credential authenticates the script
download path and the API bearer header.

- internal/token: derivation + hashing; migration 0006 adds token_epoch
- seed refreshes the owner's epoch-0 hash only before first rotation
- httpmw.Auth resolves the acting Reader from the credential hash and
  stashes it in the request context; the retired API_TOKEN resolves to
  the owner until API_TOKEN_GRACE_UNTIL, logged per use, on both the
  bearer and script-download paths
- userscript handler renders the bindmounted file with the resolved
  Reader's credential substituted for __API_TOKEN__; a legacy-path
  request during grace serves the derived credential, so devices
  self-migrate on their next update poll
- web UI: Userscripts panel with session-gated install endpoints that
  render the script directly (credential never in markup, address bar
  or a redirect) and confirm-gated rotation; atomic epoch bump + hash
  rewrite in the store
- both userscripts carry __API_TOKEN__ placeholders; the committed
  global-token literal is removed (rotating at deploy retires it for
  real — it survives in git history)
- env: TOKEN_KEY required, API_TOKEN/API_TOKEN_GRACE_UNTIL retire the
  legacy credential; docs and compose updated

backend/internal/token/token_test.go

@@ -0,0 +1,53 @@
+package token
+
+import (
+	"bytes"
+	"crypto/sha256"
+	"testing"
+)
+
+func TestTokenDeterministicPerReaderAndEpoch(t *testing.T) {
+	key := []byte("deployment-secret")
+	a := Token(key, "reader-1", 0)
+	b := Token(key, "reader-1", 0)
+	if a != b {
+		t.Fatal("same (reader, epoch) derived different credentials")
+	}
+	if a == Token(key, "reader-2", 0) {
+		t.Fatal("different readers derived the same credential")
+	}
+	if a == Token(key, "reader-1", 1) {
+		t.Fatal("rotation epoch derived the same credential")
+	}
+}
+
+func TestTokenChangesWithSecret(t *testing.T) {
+	a := Token([]byte("key-1"), "reader-1", 0)
+	b := Token([]byte("key-2"), "reader-1", 0)
+	if a == b {
+		t.Fatal("different secrets derived the same credential")
+	}
+}
+
+func TestTokenFormat(t *testing.T) {
+	cred := Token([]byte("key"), "reader-1", 0)
+	// 32 bytes of HMAC-SHA256, hex-encoded: the length the install URL and
+	// the committed placeholder both assume.
+	if len(cred) != 64 {
+		t.Fatalf("credential length = %d, want 64", len(cred))
+	}
+	for _, c := range cred {
+		if !(c >= '0' && c <= '9' || c >= 'a' && c <= 'f') {
+			t.Fatalf("credential contains non-hex byte %q", c)
+		}
+	}
+}
+
+func TestHashIsSha256OfCredential(t *testing.T) {
+	cred := Token([]byte("key"), "reader-1", 0)
+	got := Hash(cred)
+	want := sha256.Sum256([]byte(cred))
+	if !bytes.Equal(got[:], want[:]) {
+		t.Fatal("Hash is not the SHA-256 of the credential")
+	}
+}