docs: comprehensive documentation overhaul

Add complete documentation structure:
- Frontend architecture documentation
- Database schema documentation
- Deployment guides (Docker, Traefik, Authentik, Environment)
- Development guides (Setup, Testing, Contributing, Quality Gates)
- Deployment architecture documentation
- Updated docs README with complete navigation

All new features and APIs are now documented.
Quality gates: docs only, no code changes

docs/features/ssh-keys.md

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+# SSH Key Management
+
+## Overview
+
+Manage SSH key pairs for authenticating with git remotes. Keys are generated and stored securely, with the private key encrypted.
+
+## How to Use
+
+### Generating a Key Pair
+
+1. Navigate to **Settings** → **SSH Keys**
+2. Click **"Generate New Key"**
+3. Enter a **name** for the key (e.g., "GitHub Work Account")
+4. Click **"Generate"**
+
+The system will:
+- Generate an Ed25519 key pair
+- Encrypt the private key
+- Store both keys securely
+- Display the public key
+
+### Copying Public Key
+
+1. Find the key in the list
+2. Click the **"Copy"** button next to the public key
+3. Paste into your git host (GitHub, GitLab, etc.)
+
+### Adding to Git Hosts
+
+#### GitHub
+1. Go to Settings → SSH and GPG keys
+2. Click "New SSH key"
+3. Paste the public key
+4. Give it a title
+5. Click "Add SSH key"
+
+#### GitLab
+1. Go to Preferences → SSH Keys
+2. Paste the public key
+3. Set expiration (optional)
+4. Click "Add key"
+
+### Deleting Keys
+
+1. Find the key in the list
+2. Click the **"Delete"** button
+3. Confirm deletion
+
+**Note:** Deleting a key removes both public and private keys. This cannot be undone.
+
+## Security
+
+- **Ed25519 algorithm**: Modern, secure key type
+- **Encrypted storage**: Private keys are encrypted at rest
+- **No export**: Private keys cannot be exported
+- **One-way generation**: Keys are generated server-side, never transmitted
+
+## API Reference
+
+### Endpoints
+
+- `GET /ssh-keys` - List SSH keys
+- `POST /ssh-keys` - Generate new key
+- `DELETE /ssh-keys/{id}` - Delete key
+
+See [SSH Keys API](../api/ssh-keys.md) for detailed endpoint documentation.
+
+## Related Features
+
+- [Git Repositories](repositories.md) - Use SSH keys for repository access