feat: add observability stack and background task infrastructure

Add OpenTelemetry instrumentation with distributed tracing and metrics:
- Structured JSON logging with trace context correlation
- Auto-instrumentation for FastAPI, asyncpg, httpx, redis
- OTLP exporter for traces and Prometheus metrics endpoint

Implement Celery worker and notification task system:
- Celery app with Redis/SQS broker support and configurable queues
- Notification tasks for incident fan-out, webhooks, and escalations
- Pluggable TaskQueue abstraction with in-memory driver for testing

Add Grafana observability stack (Loki, Tempo, Prometheus, Grafana):
- OpenTelemetry Collector for receiving OTLP traces and logs
- Tempo for distributed tracing backend
- Loki for log aggregation with Promtail DaemonSet
- Prometheus for metrics scraping with RBAC configuration
- Grafana with pre-provisioned datasources and API overview dashboard
- Helm templates for all observability components

Enhance application infrastructure:
- Global exception handlers with structured ErrorResponse schema
- Request logging middleware with timing metrics
- Health check updated to verify task queue connectivity
- Non-root user in Dockerfile for security
- Init containers in Helm deployments for dependency ordering
- Production Helm values with autoscaling and retention policies

app/schemas/__init__.py

@@ -8,7 +8,7 @@ from app.schemas.auth import (
     SwitchOrgRequest,
     TokenResponse,
 )
-from app.schemas.common import CursorParams, PaginatedResponse
+from app.schemas.common import CursorParams, ErrorDetail, ErrorResponse, PaginatedResponse
 from app.schemas.incident import (
     CommentRequest,
     IncidentCreate,
@@ -35,6 +35,8 @@ __all__ = [
     "TokenResponse",
     # Common
     "CursorParams",
+    "ErrorDetail",
+    "ErrorResponse",
     "PaginatedResponse",
     # Incident
     "CommentRequest",