Register the broker wait before the first poll (closes #157)

GET /api/v1/messages read the client output queue and only then
registered with the broker. Notify wakes only the channels registered
at that moment, so a message queued between the empty read and the
registration waited out the whole long-poll timeout. The handler now
registers first and removes its channel on every exit path; the IRC
relay loop registers before each drain in the same way.

The handlers read the queue through a function field that defaults to
Database.PollMessages, so tests can act just after a read: one checks
that a message queued there comes back at once, others that no wait
channel stays registered however a request ends.

Model: opus-5-5

internal/handlers/export_test.go

@@ -0,0 +1,19 @@
+package handlers
+
+import (
+	"context"
+
+	"sneak.berlin/go/neoirc/internal/db"
+)
+
+// SetPollMessages makes HandleGetMessages read the client
+// output queue with poll in place of Database.PollMessages.
+func (hdlr *Handlers) SetPollMessages(
+	poll func(
+		ctx context.Context,
+		clientID, afterQueueID int64,
+		limit int,
+	) ([]db.IRCMessage, int64, error),
+) {
+	hdlr.pollMessages = poll
+}