fix: align Terraform DB config and reorganize docs (#33)

* fix: align database.tf backup config with actual Cloud SQL instance state

Backup was disabled on the real instance; syncing so terraform plan is clean
with no spurious diff on next apply.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* docs: reorganize README into focused docs/ files

README trimmed to a lean landing page. Detailed content moved to:
- docs/ARCHITECTURE.md (BFF pattern, ReAct agent, security model)
- docs/INFRASTRUCTURE.md (Cloud topology, Terraform, Secret Manager, CI/CD)
- docs/AZURE_SETUP.md (Entra ID registration, Cosmos DB, frontend config)
- docs/DEVELOPMENT.md (local setup, testing, code style)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ChingEnLin