feat: implement SSH key management

- Add backend API endpoints for SSH key CRUD (POST, GET, DELETE)
- Implement Ed25519 key generation with Fernet-encrypted private keys
- Add frontend SSH keys page with generate, list, and delete functionality
- Include copy-to-clipboard for public keys
- Add responsive CSS styles for key cards
- Register ssh_keys router in main.py
- Add basic auth tests for SSH key endpoints

Quality gates: ruff ✓, mypy ✓, typecheck ✓, lint ✓

openspec/changes/ssh-key-management/specs/ssh-key-management/spec.md

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+## ADDED Requirements
+
+### Requirement: SSH Key Generation
+
+The system SHALL generate Ed25519 SSH key pairs and encrypt the private key with Fernet.
+
+#### Scenario: Generate key
+- **WHEN** an authenticated user requests a new SSH key with a name
+- **THEN** an Ed25519 key pair is generated
+- **AND** the private key is encrypted with Fernet
+- **AND** the public key is stored in OpenSSH format
+- **AND** the key is associated with the user
+
+### Requirement: SSH Key Listing
+
+The system SHALL list all SSH keys for the authenticated user.
+
+#### Scenario: List keys
+- **WHEN** an authenticated user views their SSH keys
+- **THEN** all their keys are listed with name, public key preview, and created date
+
+### Requirement: SSH Key Display
+
+The system SHALL display public keys for copying.
+
+#### Scenario: Copy public key
+- **WHEN** an authenticated user views an SSH key
+- **THEN** the full public key is displayed in OpenSSH format
+- **AND** a copy-to-clipboard button is available
+
+### Requirement: SSH Key Deletion
+
+The system SHALL support key removal.
+
+#### Scenario: Delete key
+- **WHEN** an authenticated user deletes an SSH key
+- **THEN** it's removed from the database
+- **AND** the key files are deleted if stored on disk