Add Azure Monitor Health Model as Bicep module

Adds a health model (Microsoft.CloudHealth/healthModels) that deploys
alongside the existing infrastructure via azd. The module:

- Creates a dedicated managed identity with Monitoring Reader + Reader roles
- Organizes entities by use case: RAG Chat, Document Ingestion, Platform, Observability
- Conditionally creates entities based on deployment flags (deploymentTarget,
  deployAzureOpenAi, useApplicationInsights, useSpeechOutputAzure, etc.)
- Includes 31 signal definitions covering:
  - OpenAI: availability, latency, throttle, tokens (input/output split),
    client errors (context-too-long), rate limiting, content policy blocks
  - App Performance: failed requests, exceptions, response time,
    dependency failures, dependency duration
  - Search: latency, QPS, throttle, documents processed, index storage
  - Container Apps: CPU%, memory%, restarts, request volume
  - Storage: transactions, latency
  - Document Intelligence: success rate, latency, errors, pages processed
  - Platform: ACA ingress CPU
  - Observability: telemetry trace volume
- Computes canvas layout positions with Bicep arithmetic (no overlaps)
- All resource IDs parameterized, no hardcoded subscriptions or names

Files changed:
  infra/core/monitor/health-model.bicep  (new)
  infra/main.bicep                       (module call + useHealthModel param)
  infra/main.parameters.json             (AZURE_USE_HEALTH_MODEL env var)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: abossard