MVP: IRC envelope format, long-polling, per-client queues, SPA rewrite

Major changes:
- Consolidated schema into single migration with IRC envelope format
- Messages table stores command/from/to/body(JSON)/meta(JSON) per spec
- Per-client delivery queues (client_queues table) with fan-out
- In-memory broker for long-poll notifications (no busy polling)
- GET /messages supports ?after=<queue_id>&timeout=15 long-polling
- All commands (JOIN/PART/NICK/TOPIC/QUIT/PING) broadcast events
- Channels are ephemeral (deleted when last member leaves)
- PRIVMSG to nicks (DMs) fan out to both sender and recipient
- SPA rewritten in vanilla JS (no build step needed):
  - Long-poll via recursive fetch (not setInterval)
  - IRC envelope parsing with system message display
  - /nick, /join, /part, /msg, /quit commands
  - Unread indicators on inactive tabs
  - DM tabs from user list clicks
- Removed unused models package (was for UUID-based schema)
- Removed conflicting UUID-based db methods
- Increased HTTP write timeout to 60s for long-poll support

internal/server/http.go

@@ -4,6 +4,6 @@ import "time"
 
 const (
 	httpReadTimeout  = 10 * time.Second
-	httpWriteTimeout = 10 * time.Second
+	httpWriteTimeout = 60 * time.Second
 	maxHeaderBytes   = 1 << 20
 )