feat: add OTel + Prometheus instrumentation for Guard/RAG observability

[Pcmhacker-piro]

Summary
Adds OpenTelemetry + Prometheus instrumentation across the backend for comprehensive observability of Guard inference, RAG retrieval, and database queries.
Closes #319
Type of Change

• New feature
  Checklist
• I have read CONTRIBUTING.md
• My code follows the project style
• I have added/updated tests where relevant
• Tests/lint pass locally (if available)
• I have not committed .env or any secrets
• I have updated documentation if needed
  What was added
• FastAPI middleware instrumentation -- request latency histograms, active request gauges, and request count via prometheus-fastapi-instrumentator
• Guard inference pipeline -- aegis_guard_inference_duration_seconds histogram and aegis_guard_scans_total counter, labelled by decision (allow/block/sanitize)
• RAG retrieval chain -- aegis_rag_retrieval_duration_seconds histogram and aegis_rag_queries_total counter
• SQLAlchemy query instrumentation -- aegis_db_query_duration_seconds histogram via cursor event listeners, labelled by SQL operation type
• /metrics -- Prometheus scrape endpoint (auto-exposed by instrumentator)
• /ready -- Readiness probe endpoint
• OTel configuration settings -- OTEL_SERVICE_NAME, OTEL_METRICS_EXPORTER, OTEL_TRACES_EXPORTER
• Dependencies -- added opentelemetry-api, opentelemetry-sdk, opentelemetry-exporter-prometheus, prometheus-fastapi-instrumentator
  CHANGED FILES
• backend/app/core/telemetry.py (new) — Core OTel + Prometheus module with metric definitions, decorators, and instrumentator setup
• backend/app/core/config.py — Added OTEL_* settings
• backend/app/core/database.py — SQLAlchemy before/after cursor execute event listeners for query latency
• backend/app/main.py — Wired up setup_telemetry(), added /ready endpoint
• backend/app/modules/guard/llm_guard.py — Added @instrument_guard decorator to LLMGuard.guard()
• backend/app/modules/rag/retrieval_chain.py — Added @instrument_rag decorator to GroundedRetrievalQA.call()
• backend/requirements.txt — Added OTel + prometheus-fastapi-instrumentator deps; removed duplicate entries
  COMMITS
  72e9547 - feat: add OTel + Prometheus instrumentation for Guard/RAG observability
  TESTING PERFORMED
  N/A (dependencies not installed locally; CI will run tests with updated requirements.txt)
  FINAL STATUS
• Branch Name: fix/otel-prometheus-observability
• Commit Hash: 72e9547
• PR Created: Compare URL ready — see link above (token lacked createPullRequest scope)
• Ready for Review: Yes

[Pcmhacker-piro]

@SdSarthak

Hi, the checks have passed. Could you please review and approve the pending workflows when you have a chance? Thank you!