docs: update README schema section to match actual database schema

Update the Schema section and related references throughout README.md to
accurately reflect the current 001_initial.sql migration:

- Rename 'users' table to 'sessions' with new columns: uuid, password_hash,
  signing_key, away_message
- Add new 'clients' table (uuid, session_id FK, token, created_at, last_seen)
- Add topic_set_by and topic_set_at columns to 'channels' table
- Update channel_members FK from user_id to session_id
- Add params column to messages table
- Update client_queues FK from user_id to client_id
- Update Queue Architecture diagram labels and surrounding text
- Update In-Memory Broker description to use client_id terminology
- Update Multi-Client Model MVP note to reflect sessions/clients split

README.md

@@ -301,8 +301,8 @@ The server implements HTTP long-polling for real-time message delivery:
    - The client disconnects (connection closed, no response needed)
 
 **Implementation detail:** The server maintains an in-memory broker with
-per-user notification channels. When a message is enqueued for a user, the
-broker closes all waiting channels for that user, waking up any blocked
+per-client notification channels. When a message is enqueued for a client, the
+broker closes all waiting channels for that client, waking up any blocked
 long-poll handlers. This is O(1) notification — no polling loops, no database
 scanning.
 
@@ -460,28 +460,28 @@ The entire read/write loop for a client is two endpoints. Everything else
               │              │              │
     ┌─────────▼──┐  ┌───────▼────┐  ┌──────▼─────┐
     │client_queue│  │client_queue│  │client_queue│
-    │ user_id=1  │  │ user_id=2  │  │ user_id=3  │
+    │ client_id=1│  │ client_id=2│  │ client_id=3│
     │ msg_id=N   │  │ msg_id=N   │  │ msg_id=N   │
     └────────────┘  └────────────┘  └────────────┘
          alice           bob             carol
 
-Each message is stored ONCE. One queue entry per recipient.
+Each message is stored ONCE. One queue entry per recipient client.
 ```
 
-The `client_queues` table contains `(user_id, message_id)` pairs. When a
+The `client_queues` table contains `(client_id, message_id)` pairs. When a
 client polls with `GET /messages?after=<queue_id>`, the server queries for
-queue entries with `id > after` for that user, joins against the messages
+queue entries with `id > after` for that client, joins against the messages
 table, and returns the results. The `queue_id` (auto-incrementing primary
 key of `client_queues`) serves as a monotonically increasing cursor.
 
 ### In-Memory Broker
 
 The server maintains an in-memory notification broker to avoid database
-polling. The broker is a map of `user_id → []chan struct{}`. When a message
-is enqueued for a user:
+polling. The broker is a map of `client_id → []chan struct{}`. When a message
+is enqueued for a client:
 
-1. The handler calls `broker.Notify(userID)`
-2. The broker closes all waiting channels for that user
+1. The handler calls `broker.Notify(clientID)`
+2. The broker closes all waiting channels for that client
 3. Any goroutines blocked in `select` on those channels wake up
 4. The woken handler queries the database for new queue entries
 5. Messages are returned to the client
@@ -1937,44 +1937,50 @@ The database schema is managed via embedded SQL migration files in
 
 #### `sessions`
 | Column          | Type     | Description |
-|----------------|----------|-------------|
+|-----------------|----------|-------------|
 | `id`            | INTEGER  | Primary key (auto-increment) |
 | `uuid`          | TEXT     | Unique session UUID |
 | `nick`          | TEXT     | Unique nick |
-| `password_hash`| TEXT     | bcrypt hash (empty string for anonymous sessions) |
+| `password_hash` | TEXT     | bcrypt hash (empty string for anonymous sessions) |
 | `signing_key`   | TEXT     | Public signing key (empty string if unset) |
 | `away_message`  | TEXT     | Away message (empty string if not away) |
 | `created_at`    | DATETIME | Session creation time |
 | `last_seen`     | DATETIME | Last API request time |
 
+Index on `(uuid)`.
+
 #### `clients`
 | Column       | Type     | Description |
-|-------------|----------|-------------|
+|--------------|----------|-------------|
 | `id`         | INTEGER  | Primary key (auto-increment) |
 | `uuid`       | TEXT     | Unique client UUID |
-| `session_id`| INTEGER  | FK → sessions.id (cascade delete) |
+| `session_id` | INTEGER  | FK → sessions.id (cascade delete) |
 | `token`      | TEXT     | Unique auth token (SHA-256 hash of 64 hex chars) |
-| `created_at`| DATETIME | Client creation time |
+| `created_at` | DATETIME | Client creation time |
 | `last_seen`  | DATETIME | Last API request time |
 
+Indexes on `(token)` and `(session_id)`.
+
 #### `channels`
 | Column        | Type     | Description |
-|-------------|----------|-------------|
+|---------------|----------|-------------|
 | `id`          | INTEGER  | Primary key (auto-increment) |
 | `name`        | TEXT     | Unique channel name (e.g., `#general`) |
 | `topic`       | TEXT     | Channel topic (default empty) |
-| `created_at`| DATETIME | Channel creation time |
-| `updated_at`| DATETIME | Last modification time |
+| `topic_set_by`| TEXT     | Nick of the user who set the topic (default empty) |
+| `topic_set_at`| DATETIME | When the topic was last set |
+| `created_at`  | DATETIME | Channel creation time |
+| `updated_at`  | DATETIME | Last modification time |
 
 #### `channel_members`
 | Column       | Type     | Description |
-|-------------|----------|-------------|
+|--------------|----------|-------------|
 | `id`         | INTEGER  | Primary key (auto-increment) |
-| `channel_id`| INTEGER  | FK → channels.id |
-| `user_id`   | INTEGER  | FK → users.id |
+| `channel_id` | INTEGER  | FK → channels.id (cascade delete) |
+| `session_id` | INTEGER  | FK → sessions.id (cascade delete) |
 | `joined_at`  | DATETIME | When the user joined |
 
-Unique constraint on `(channel_id, user_id)`.
+Unique constraint on `(channel_id, session_id)`.
 
 #### `messages`
 | Column      | Type     | Description |
@@ -1984,6 +1990,7 @@ Unique constraint on `(channel_id, user_id)`.
 | `command`   | TEXT     | IRC command (`PRIVMSG`, `JOIN`, etc.) |
 | `msg_from`  | TEXT     | Sender nick |
 | `msg_to`    | TEXT     | Target (`#channel` or nick) |
+| `params`    | TEXT     | JSON-encoded IRC-style positional parameters |
 | `body`      | TEXT     | JSON-encoded body (array or object) |
 | `meta`      | TEXT     | JSON-encoded metadata |
 | `created_at`| DATETIME | Server timestamp |
@@ -1994,11 +2001,11 @@ Indexes on `(msg_to, id)` and `(created_at)`.
 | Column      | Type     | Description |
 |-------------|----------|-------------|
 | `id`        | INTEGER  | Primary key (auto-increment). Used as the poll cursor. |
-| `user_id`   | INTEGER  | FK → users.id |
-| `message_id`| INTEGER  | FK → messages.id |
+| `client_id` | INTEGER  | FK → clients.id (cascade delete) |
+| `message_id`| INTEGER  | FK → messages.id (cascade delete) |
 | `created_at`| DATETIME | When the entry was queued |
 
-Unique constraint on `(user_id, message_id)`. Index on `(user_id, id)`.
+Unique constraint on `(client_id, message_id)`. Index on `(client_id, id)`.
 
 The `client_queues.id` is the monotonically increasing cursor used by
 `GET /messages?after=<id>`. This is more reliable than timestamps (no clock