chore: comment cleanup pass ahead of open-sourcing

Removes leftover debug scaffolding (stray Console.WriteLine/NSLog traces,
dead nick_buf_ptr, a no-op --print-config flag now implemented for real),
fixes stale/misleading comments (channel passwords are no longer a "future
M5+" feature, a wrong cross-reference, a stale TlsContext::close() mention,
an incomplete BanRecord::subject_type doc, and a smoke test pointing at a
build/m1-dev preset that no longer exists), strips internal M1-M5 milestone
jargon from comments now that the roadmap is done, trims comments that just
restated the following line, and consolidates a few "why" explanations that
were duplicated 2-3 times in the same file.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

server/src/db.cpp

@@ -89,7 +89,6 @@ bool Database::open(std::string& error) {
 }
 
 bool Database::migrate(std::string& error) {
-    // Ensure server_meta row exists.
     if (!exec("INSERT OR IGNORE INTO server_meta (key, value) VALUES ('schema_version', '1')",
               error))
         return false;
@@ -241,7 +240,6 @@ std::optional<Account> Database::authenticate(const std::string& username,
     if (crypto_pwhash_str_verify(hash.c_str(), password.c_str(), password.size()) != 0)
         return std::nullopt;
 
-    // Update last_login
     int64_t now = now_unix();
     sqlite3_stmt* upd = nullptr;
     sqlite3_prepare_v2(db_, "UPDATE accounts SET last_login=? WHERE id=?", -1, &upd, nullptr);