feat: add username/hostname support with IRC hostmask format

- Add username and hostname columns to sessions table (001_initial.sql)
- Accept optional username field in session creation and registration
  endpoints; defaults to nick if not provided
- Resolve hostname via reverse DNS of connecting client IP at session
  creation time (supports X-Forwarded-For and X-Real-IP headers)
- Display real username and hostname in WHOIS (311 RPL_WHOISUSER) and
  WHO (352 RPL_WHOREPLY) responses instead of nick/servername
- Add FormatHostmask helper for nick!user@host format
- Add SessionHostInfo type and GetSessionHostInfo query
- Include username/hostname in MemberInfo and ChannelMembers results
- Extract validateHashcash and resolveUsername helpers to stay under
  funlen limits
- Add comprehensive unit tests for all new DB functions, hostmask
  formatting, and integration tests for WHOIS/WHO responses
- Update README with hostmask documentation, new API fields, and
  updated schema reference

internal/db/schema/001_initial.sql

@@ -6,6 +6,8 @@ CREATE TABLE IF NOT EXISTS sessions (
     id INTEGER PRIMARY KEY AUTOINCREMENT,
     uuid TEXT NOT NULL UNIQUE,
     nick TEXT NOT NULL UNIQUE,
+    username TEXT NOT NULL DEFAULT '',
+    hostname TEXT NOT NULL DEFAULT '',
     password_hash TEXT NOT NULL DEFAULT '',
     signing_key TEXT NOT NULL DEFAULT '',
     away_message TEXT NOT NULL DEFAULT '',