Bring README.md in line with the modes and the schema (closes #119)

The MODE section described a query-only command. It now covers channel
and user mode queries and changes on the HTTP API and the IRC listener,
the letters each accepts, what is broadcast, and the error numerics.
The schema section now lists every table and column in
001_initial.sql.

The rest of the README was checked against the code, and each statement
the code contradicts is corrected: polled numerics carry their name in
command and their number in code; command errors are numerics with HTTP
200, not 404 or 409; the broker is keyed by session; WAL is never on; an
empty IRC_LISTEN_ADDR does not disable the listener; and the IRC
listener handles modes, INVITE, +s and +H more narrowly.

Model: opus-5-5

README.md

@@ -1266,12 +1266,6 @@ IRC listener sends the usual 3-digit code. Over the HTTP API, 005 arrives named
 | `501` | ERR_UMODEUNKNOWNFLAG | User mode not accepted                   | `{"command":"ERR_UMODEUNKNOWNFLAG","code":501,"to":"alice","body":["Unknown MODE flag"]}`                                                                                                                    |
 | `502` | ERR_USERSDONTMATCH   | User MODE for another nick               | `{"command":"ERR_USERSDONTMATCH","code":502,"to":"alice","body":["Can't change mode for other users"]}`                                                                                                      |
 
-**Note:** Numeric replies are now implemented. All IRC command responses
-(success and error) are delivered as numeric replies through the message queue.
-HTTP error codes are reserved for transport-level issues (auth failures,
-malformed requests, server errors). The `params` field in the message envelope
-carries IRC-style parameters (e.g., channel name, target nick).
-
 ### Channel Modes
 
 Inspired by IRC, simplified. See [MODE](#mode--query-and-change-modes) for how
@@ -2421,7 +2415,7 @@ applied; `001_initial.sql` creates the tables below.
 | `signing_key`   | TEXT     | Public signing key; nothing sets or reads it yet              |
 | `away_message`  | TEXT     | Away message (empty string if not away)                       |
 | `created_at`    | DATETIME | Session creation time                                         |
-| `last_seen`     | DATETIME | Last login or authenticated HTTP API request                  |
+| `last_seen`     | DATETIME | Last login or authenticated HTTP API request (or creation)    |
 
 Index on `(uuid)`.
 
@@ -3163,11 +3157,11 @@ When the limit is exceeded, the server returns **429 Too Many Requests** with a
 > targeted attack.
 
 **Why rate limits here but not on session creation?** Session creation is
-protected by hashcash proof-of-work (stateless, no IP tracking needed). Login
-involves bcrypt password verification against a registered account — a
-fundamentally different threat model where an attacker targets a specific
-account. Per-IP rate limiting is appropriate here because the cost of a wrong
-guess is borne by the server (bcrypt), not the client.
+protected by hashcash proof-of-work (no IP tracking needed). Login involves
+bcrypt password verification against a registered account — a fundamentally
+different threat model where an attacker targets a specific account. Per-IP rate
+limiting is appropriate here because the cost of a wrong guess is borne by the
+server (bcrypt), not the client.
 
 ---
 
@@ -3222,8 +3216,8 @@ guess is borne by the server (bcrypt), not the client.
       331-332 TOPIC, 352-353 WHO/NAMES, 366, 372-376 MOTD, 401-502 errors)
 - [x] **Max message size enforcement** — reject request bodies over
       `MAX_MESSAGE_SIZE` bytes
-- [x] **NOTICE command** — distinct from PRIVMSG (no RPL_AWAY, no hashcash
-      check)
+- [x] **NOTICE command** — distinct from PRIVMSG over the HTTP API (no RPL_AWAY,
+      no hashcash check)
 - [x] **Multi-client sessions** — set a password via PASS command, then login
       from additional devices via `POST /api/v1/login`
 - [x] **Cookie-based auth** — HttpOnly cookies replace Bearer tokens for all API