feat(bridge): Add pending contacts management API endpoints

Added two new HTTP endpoints to meshcore-bridge for managing pending contacts
(contacts awaiting manual approval when manual_add_contacts mode is enabled):

New endpoints:
- GET /pending_contacts - List all pending contacts awaiting approval
  - Parses meshcli output format: "Name: <hex_public_key>"
  - Returns JSON array with {name, public_key} objects
  - Includes raw_stdout for debugging

- POST /add_pending - Approve and add a pending contact
  - Accepts JSON body: {"selector": "<name_or_pubkey>"}
  - Validates selector is non-empty string
  - Executes meshcli add_pending command via persistent session

Additional changes:
- Added curl to mc-webui Dockerfile for testing endpoints
- Updated README with Testing Bridge API section
- Included example curl commands and expected responses

Implementation notes:
- Uses existing MeshCLISession.execute_command() - no new processes
- Same persistent session and command queue architecture
- Consistent error handling with existing /cli endpoint

Enables future UI for manual contact approval workflow.

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Dockerfile

@@ -3,6 +3,9 @@
 
 FROM python:3.11-slim
 
+# Install curl for testing
+RUN apt-get update && apt-get install -y curl && rm -rf /var/lib/apt/lists/*
+
 # Set working directory
 WORKDIR /app