🛡️ Sentinel: [CRITICAL] Fix authorization bypass in API /usage endpoint

[ovasylenko]

🚨 Severity: CRITICAL
💡 Vulnerability: The /usage endpoint lacked an authorization check when a caller omitted the X-Tenant-Id header (unscoped caller). It allowed any unscoped caller to arbitrarily specify a tenant ID via the ?tenant= query parameter and view that tenant's usage and cost data.
🎯 Impact: This Insecure Direct Object Reference (IDOR) / Missing Authorization vulnerability allowed unauthenticated or unauthorized users to scrape usage and cost metrics across all tenants in the system.
🔧 Fix: Updated the get_usage handler in orch8-api/src/usage.rs to require the admin_ctx extractor. If tenant_ctx is None, it now calls require_admin(&admin_ctx)? before allowing the use of the ?tenant= parameter. Also updated the MCP proxy logic in orch8-api/src/mcp_server.rs to match the new signature.
✅ Verification: cargo +nightly test -p orch8-api --test usage passes successfully, and cargo +nightly check -p orch8-api confirms no compilation errors exist.

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PR created automatically by Jules for task 13184674616619685396 started by @ovasylenko

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