docs: overhaul project documentation and contributing guide (Task #190)

docs/data-model.md

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+# 📊 Data Model
+
+Plexus uses **DuckDB** for fast, analytical, and serverless-friendly data storage.
+
+## 🗄 Tables
+
+### `users`
+Stores user identity and profile information.
+- `wallet_address` (VARCHAR, PK): Unique Solana wallet address.
+- `username` (VARCHAR, UNIQUE): Display name.
+- `bio` (VARCHAR): User biography.
+- `banner_color` (VARCHAR): Hex code for profile banner.
+- `last_seen` (TIMESTAMP): Last activity time.
+
+### `messages`
+Stores chat history.
+- `id` (INTEGER, PK): Unique message ID (from `seq_msg_id`).
+- `channel_id` (VARCHAR): ID of the channel.
+- `wallet_address` (VARCHAR, FK): Sender's wallet.
+- `content` (VARCHAR): Message text.
+- `timestamp` (TIMESTAMP): Time sent.
+- `tx_id` (VARCHAR): Simulated transaction ID.
+
+### `reactions`
+Stores message reactions.
+- `message_id` (INTEGER, FK): ID of the message.
+- `wallet_address` (VARCHAR, FK): User who reacted.
+- `emoji` (VARCHAR): The emoji character.
+- *Composite PK*: `(message_id, wallet_address, emoji)`.
+
+### `posts`
+Stores social wall posts.
+- `id` (INTEGER, PK): Unique post ID (from `seq_post_id`).
+- `wallet_address` (VARCHAR, FK): Owner of the wall.
+- `content` (VARCHAR): Post text.
+- `timestamp` (TIMESTAMP): Time posted.
+
+## 🔢 Sequences
+- `seq_msg_id`: Increments for each new message.
+- `seq_post_id`: Increments for each new wall post.
+
+## 🔄 Migrations
+The `server/db.js` file handles automatic schema initialization and migrations (e.g., adding `tx_id` or `bio` columns if they are missing from an existing database).