Refuse passwords over 72 bytes and confirm only stored ones (closes #144)

bcrypt cannot hash a password longer than 72 bytes. PASS over the HTTP
API answered one with a 500, and the IRC listener confirmed it without
storing it.

Both transports now refuse a password outside 8 to 72 bytes with the
reply a short one already got: 461 over the HTTP API, a NOTICE over the
IRC listener. setPassword returns its error, and "Password set" is sent
only when the password was stored. A PASS given before registration, an
empty one included, is checked after the welcome burst and gets the
same NOTICE.

Model: opus-5-5

README.md

@@ -180,7 +180,7 @@ For users who want to access the same session from multiple devices:
 - **Set password via IRC PASS command**: the authenticated client sends
   `POST /api/v1/messages` with `{"command":"PASS","body":["mypassword"]}`. The
   server hashes the password with bcrypt and stores it on the session. Password
-  must be at least 8 characters.
+  must be 8 to 72 bytes.
 - **Login from another client**: `POST /api/v1/login` with nick and password →
   server verifies the password, creates a new client for the existing session,
   and sets an auth cookie. Channel memberships are shared; the new client gets
@@ -835,10 +835,11 @@ Set a password on the current session, enabling multi-client login via
 
 **Behavior:**
 
-- `body[0]` is the password. Must be at least 8 characters.
+- `body[0]` is the password. Must be 8 to 72 bytes; bcrypt cannot hash a longer
+  one.
 - On success, the server responds with `{"status": "ok"}`.
-- If the password is too short or missing, the server sends ERR_NEEDMOREPARAMS
-  (461) via the message queue.
+- If the password is too short, too long or missing, the server sends
+  ERR_NEEDMOREPARAMS (461) via the message queue.
 - Calling PASS again overwrites the previous password.
 - Once a password is set, `POST /api/v1/login` can be used with the nick and
   password to create additional clients on the same session.
@@ -2678,9 +2679,11 @@ IRC_LISTEN_ADDR: ""
 
 - **Wire format**: CR-LF delimited lines, max 512 bytes per line
 - **Connection registration**: Clients must send `NICK` and `USER` to register.
-  An optional `PASS` before registration sets the session password (minimum 8
-  characters). Registering creates a session without a hashcash stamp; closing
-  the connection ends the session, as `QUIT` does.
+  An optional `PASS` before registration sets the session password (8 to 72
+  bytes); one outside that range is refused with a `NOTICE` once registration
+  completes, as `PASS` after registration is. Registering creates a session
+  without a hashcash stamp; closing the connection ends the session, as `QUIT`
+  does.
 - **PING**: a connection that sends nothing for 90 seconds is sent a `PING`, and
   is closed if it sends nothing in the 30 seconds after that; any line, such as
   the client's `PONG`, starts both waits again.