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/common.py

@@ -3,6 +3,47 @@
 from pydantic import BaseModel, Field
 
 
+class ErrorDetail(BaseModel):
+    """Individual error detail for validation errors."""
+
+    loc: list[str | int] = Field(description="Location of the error (field path)")
+    msg: str = Field(description="Error message")
+    type: str = Field(description="Error type identifier")
+
+
+class ErrorResponse(BaseModel):
+    """Structured error response returned by all error handlers."""
+
+    error: str = Field(description="Error type (e.g., 'not_found', 'validation_error')")
+    message: str = Field(description="Human-readable error message")
+    details: list[ErrorDetail] | None = Field(
+        default=None, description="Additional error details for validation errors"
+    )
+    request_id: str | None = Field(
+        default=None, description="Request trace ID for debugging"
+    )
+
+    model_config = {
+        "json_schema_extra": {
+            "examples": [
+                {
+                    "error": "not_found",
+                    "message": "Incident not found",
+                    "request_id": "abc123def456",
+                },
+                {
+                    "error": "validation_error",
+                    "message": "Request validation failed",
+                    "details": [
+                        {"loc": ["body", "title"], "msg": "Field required", "type": "missing"}
+                    ],
+                    "request_id": "abc123def456",
+                },
+            ]
+        }
+    }
+
+
 class CursorParams(BaseModel):
     """Pagination parameters using cursor-based pagination."""