Bind history to the recipient session and channel, not the name (closes #148)

History was selected by msg_to, so whoever next held a released nick, or
created a deleted channel again under its old name, could read the
earlier holder's messages. Each message row now records the session a
DM or INVITE was sent to (to_session_id) and the channel a channel
message was sent to (to_channel_id), and GET /api/v1/history selects by
the one that applies, using its index. A session's DM history now also
follows it across nick changes. A DM or channel message whose recipient
is deleted before it is stored gets the unknown nick or channel error.

Model: opus-5-5

internal/handlers/api_test.go

@@ -1551,6 +1551,60 @@ func (tserver *testServer) getHistory(
 	return resp.StatusCode, msgs
 }
 
+// getAllHistory pages back through target's history
+// pageSize messages at a time and returns the message texts,
+// oldest first, and the number of pages that held any. It
+// stops after maxPages pages, so a before that is ignored
+// cannot loop forever.
+func (tserver *testServer) getAllHistory(
+	token, target string,
+	pageSize, maxPages int,
+) ([]string, int) {
+	tserver.t.Helper()
+
+	var (
+		got   []string
+		pages int
+	)
+
+	// The first page has no before; each later one passes the
+	// id of the oldest message on the page fetched before it.
+	before := ""
+
+	for range maxPages {
+		status, page := tserver.getHistory(
+			token, target, before, pageSize,
+		)
+		if status != http.StatusOK {
+			tserver.t.Fatalf("expected 200, got %d", status)
+		}
+
+		if len(page) == 0 {
+			break
+		}
+
+		pages++
+
+		texts := make([]string, 0, len(page))
+
+		for _, msg := range page {
+			var body []string
+
+			err := json.Unmarshal(msg.Body, &body)
+			if err != nil {
+				tserver.t.Fatalf("decode body: %v", err)
+			}
+
+			texts = append(texts, body...)
+		}
+
+		got = slices.Insert(got, 0, texts...)
+		before = page[0].ID
+	}
+
+	return got, pages
+}
+
 func TestHistoryPaging(t *testing.T) {
 	const (
 		channel   = "#paging"
@@ -1579,46 +1633,10 @@ func TestHistoryPaging(t *testing.T) {
 		})
 	}
 
-	var (
-		got   []string
-		pages int
+	got, pages := tserver.getAllHistory(
+		token, channel, pageSize, msgCount,
 	)
 
-	// The first page has no before; each later one passes the
-	// id of the oldest message on the page fetched before it.
-	before := ""
-
-	for range msgCount {
-		status, page := tserver.getHistory(
-			token, channel, before, pageSize,
-		)
-		if status != http.StatusOK {
-			t.Fatalf("expected 200, got %d", status)
-		}
-
-		if len(page) == 0 {
-			break
-		}
-
-		pages++
-
-		texts := make([]string, 0, len(page))
-
-		for _, msg := range page {
-			var body []string
-
-			err := json.Unmarshal(msg.Body, &body)
-			if err != nil {
-				t.Fatalf("decode body: %v", err)
-			}
-
-			texts = append(texts, body...)
-		}
-
-		got = slices.Insert(got, 0, texts...)
-		before = page[0].ID
-	}
-
 	if pages != wantPages {
 		t.Fatalf(
 			"expected %d pages, got %d", wantPages, pages,
@@ -1630,6 +1648,81 @@ func TestHistoryPaging(t *testing.T) {
 	}
 }
 
+// TestHistoryDMPaging pages through a nick's DM history,
+// which holds the DMs its session got under an earlier nick
+// and none sent to another session, and checks that the id
+// of a DM sent to another session is not accepted as before.
+func TestHistoryDMPaging(t *testing.T) {
+	const (
+		firstNick = "readerFirst"
+		laterNick = "readerLater"
+		otherNick = "otherReader"
+		// DMs sent to firstNick before the nick change.
+		sentEarly = 2
+		pageSize  = 2
+		wantPages = 3
+	)
+
+	tserver := newTestServer(t)
+	senderToken := tserver.createSession("dmPager")
+	readerToken := tserver.createSession(firstNick)
+	otherToken := tserver.createSession(otherNick)
+
+	sendDM := func(to, text string) {
+		tserver.sendCommand(senderToken, map[string]any{
+			commandKey: privmsgCmd,
+			toKey:      to,
+			bodyKey:    []string{text},
+		})
+	}
+
+	want := []string{"dm 1", "dm 2", "dm 3", "dm 4", "dm 5"}
+
+	for _, text := range want[:sentEarly] {
+		sendDM(firstNick, text)
+	}
+
+	_, result := tserver.sendCommand(readerToken, map[string]any{
+		commandKey: irc.CmdNick,
+		bodyKey:    []string{laterNick},
+	})
+	if result[nickKey] != laterNick {
+		t.Fatalf("expected nick %s, got %v", laterNick, result)
+	}
+
+	sendDM(otherNick, "not for the reader")
+
+	for _, text := range want[sentEarly:] {
+		sendDM(laterNick, text)
+	}
+
+	got, pages := tserver.getAllHistory(
+		readerToken, laterNick, pageSize, len(want),
+	)
+
+	if pages != wantPages {
+		t.Fatalf(
+			"expected %d pages, got %d", wantPages, pages,
+		)
+	}
+
+	if !slices.Equal(got, want) {
+		t.Fatalf("expected %q, got %q", want, got)
+	}
+
+	_, otherDMs := tserver.getHistory(otherToken, otherNick, "", 1)
+	if len(otherDMs) != 1 {
+		t.Fatalf("expected 1 DM, got %d", len(otherDMs))
+	}
+
+	status, _ := tserver.getHistory(
+		readerToken, laterNick, otherDMs[0].ID, pageSize,
+	)
+	if status != http.StatusBadRequest {
+		t.Fatalf("expected 400, got %d", status)
+	}
+}
+
 func TestHistoryUnknownBefore(t *testing.T) {
 	tserver := newTestServer(t)
 	token := tserver.createSession("lost")