feat: Initial Hub implementation

Complete LetsBe Hub service for license management and telemetry:

- Client and Instance CRUD APIs
- License key generation and validation (lb_inst_ format)
- Hub API key generation (hk_ format) for telemetry auth
- Instance activation endpoint
- Telemetry collection with privacy-first redactor
- Key rotation and suspend/reactivate functionality
- Alembic migrations for PostgreSQL
- Docker Compose deployment ready
- Comprehensive test suite

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

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

app/dependencies/__init__.py

@@ -0,0 +1,5 @@
+"""Hub dependencies."""
+
+from app.dependencies.admin_auth import validate_admin_key
+
+__all__ = ["validate_admin_key"]

app/dependencies/admin_auth.py

@@ -0,0 +1,28 @@
+"""Admin authentication dependency."""
+
+import secrets
+from typing import Annotated
+
+from fastapi import Header, HTTPException, status
+
+from app.config import settings
+
+
+def validate_admin_key(
+    x_admin_api_key: Annotated[str, Header(description="Admin API key")],
+) -> str:
+    """
+    Validate the admin API key.
+
+    Uses constant-time comparison to prevent timing attacks.
+    """
+    if not secrets.compare_digest(x_admin_api_key, settings.ADMIN_API_KEY):
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail="Invalid admin API key",
+        )
+    return x_admin_api_key
+
+
+# Type alias for dependency injection
+AdminKeyDep = Annotated[str, validate_admin_key]