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/db/schema/001_initial.sql

@@ -44,6 +44,7 @@ CREATE TABLE IF NOT EXISTS channels (
     is_topic_locked INTEGER NOT NULL DEFAULT 1,
     is_invite_only INTEGER NOT NULL DEFAULT 0,
     is_secret INTEGER NOT NULL DEFAULT 0,
+    is_no_external INTEGER NOT NULL DEFAULT 1,
     channel_key TEXT NOT NULL DEFAULT '',
     user_limit INTEGER NOT NULL DEFAULT 0,
     created_at DATETIME DEFAULT CURRENT_TIMESTAMP,