fix: address 3 blocking review findings for IRC protocol listener

1. ISUPPORT/applyChannelModes: extend IRC MODE handler to support +i/-i,
   +s/-s, +n/-n (routed through svc.SetChannelFlag), and +H/-H (hashcash
   bits with parameter parsing). Add 'n' (no external messages) as a
   proper DB-backed channel flag with is_no_external column (default: on).
   Update IRC ISUPPORT to CHANMODES=,,H,imnst to match actual support.

2. QueryChannelMode: rewrite to return complete mode string including all
   boolean flags (n, i, m, s, t) and parameterized modes (k, l, H),
   matching the HTTP handler's buildChannelModeString logic. Simplify
   buildChannelModeString to delegate to QueryChannelMode for consistency.

3. Service struct encapsulation: change exported fields (DB, Broker,
   Config, Log) to unexported (db, broker, config, log). Add NewTestService
   constructor for use by external test packages. Update ircserver
   export_test.go to use the new constructor.

Closes #89

internal/handlers/api.go

@@ -2016,62 +2016,14 @@ func (hdlr *Handlers) handleChannelMode(
 }
 
 // buildChannelModeString constructs the current mode
-// string for a channel, including +n (always on), +t, +m,
-// +i, +s, +k, +l, and +H with their parameters.
+// string for a channel by delegating to the service
+// layer's QueryChannelMode, which returns the complete
+// mode string including all flags and parameters.
 func (hdlr *Handlers) buildChannelModeString(
 	ctx context.Context,
 	chID int64,
 ) string {
-	modes := "+n"
-
-	isInviteOnly, ioErr := hdlr.params.Database.
-		IsChannelInviteOnly(ctx, chID)
-	if ioErr == nil && isInviteOnly {
-		modes += "i"
-	}
-
-	isModerated, modErr := hdlr.params.Database.
-		IsChannelModerated(ctx, chID)
-	if modErr == nil && isModerated {
-		modes += "m"
-	}
-
-	isSecret, secErr := hdlr.params.Database.
-		IsChannelSecret(ctx, chID)
-	if secErr == nil && isSecret {
-		modes += "s"
-	}
-
-	isTopicLocked, tlErr := hdlr.params.Database.
-		IsChannelTopicLocked(ctx, chID)
-	if tlErr == nil && isTopicLocked {
-		modes += "t"
-	}
-
-	var modeParams string
-
-	key, keyErr := hdlr.params.Database.
-		GetChannelKey(ctx, chID)
-	if keyErr == nil && key != "" {
-		modes += "k"
-		modeParams += " " + key
-	}
-
-	limit, limErr := hdlr.params.Database.
-		GetChannelUserLimit(ctx, chID)
-	if limErr == nil && limit > 0 {
-		modes += "l"
-		modeParams += " " + strconv.Itoa(limit)
-	}
-
-	bits, bitsErr := hdlr.params.Database.
-		GetChannelHashcashBits(ctx, chID)
-	if bitsErr == nil && bits > 0 {
-		modes += "H"
-		modeParams += " " + strconv.Itoa(bits)
-	}
-
-	return modes + modeParams
+	return hdlr.svc.QueryChannelMode(ctx, chID)
 }
 
 // queryChannelMode sends RPL_CHANNELMODEIS and