feat: add subscription validation to CLI authentication (SPEC-13 Phase 2)

Implements subscription validation for CLI cloud login flow to ensure
only users with active subscriptions can access cloud features.

Changes:
- Add SubscriptionRequiredError exception for 403 subscription errors
- Update login flow to verify subscription before enabling cloud mode
- Support both FastAPI nested and flat error response formats
- Add comprehensive test coverage (6 tests, all passing)
- Update documentation with subscription prerequisites

Error Handling:
- Clean, user-friendly error messages (no traceback)
- Subscribe URL prominently displayed
- Clear next steps for users

Validation:
- Tested with unauthorized user (blocked as expected)
- Tested with authorized user (full access granted)
- All tests passing, zero type errors

Related: SPEC-13 CLI Authentication with Subscription Validation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: phernandez

docs/cloud-cli.md

@@ -12,6 +12,15 @@ The cloud CLI enables you to:
 - **Integrity verification** with `bm cloud check`
 - **Automatic project creation** from local directories
 
+## Prerequisites
+
+Before using Basic Memory Cloud, you need:
+
+- **Active Subscription**: An active Basic Memory Cloud subscription is required to access cloud features
+- **Subscribe**: Visit [https://basicmemory.com/subscribe](https://basicmemory.com/subscribe) to sign up
+
+If you attempt to log in without an active subscription, you'll receive a "Subscription Required" error with a link to subscribe.
+
 ## The Cloud Mode Paradigm
 
 Basic Memory Cloud follows the **Dropbox/iCloud model** - a single cloud space containing all your projects, not per-project connections.
@@ -474,6 +483,26 @@ bm cloud logout
 bm cloud login
 ```
 
+### Subscription Issues
+
+**Problem**: "Subscription Required" error when logging in
+
+**Solution**: You need an active Basic Memory Cloud subscription to use cloud features.
+
+1. Visit the subscribe URL shown in the error message
+2. Sign up for a subscription
+3. Once your subscription is active, run `bm cloud login` again
+
+**Problem**: "Subscription Required" error for existing user
+
+**Solution**: Your subscription may have expired or been cancelled.
+
+1. Check your subscription status at [https://basicmemory.com/account](https://basicmemory.com/account)
+2. Renew your subscription if needed
+3. Run `bm cloud login` again
+
+Note: Access is immediately restored when your subscription becomes active.
+
 ### Sync Issues
 
 **Problem**: "Bisync not initialized"