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/db.py

@@ -6,7 +6,6 @@ from contextvars import ContextVar
 
 import asyncpg
 from asyncpg.pool import PoolConnectionProxy
-import redis.asyncio as redis
 
 
 class Database:
@@ -46,34 +45,8 @@ class Database:
                 yield conn
 
 
-class RedisClient:
-    """Manages Redis connection."""
-
-    client: redis.Redis | None = None
-
-    async def connect(self, url: str) -> None:
-        """Create Redis connection."""
-        self.client = redis.from_url(url, decode_responses=True)
-
-    async def disconnect(self) -> None:
-        """Close Redis connection."""
-        if self.client:
-            await self.client.aclose()
-
-    async def ping(self) -> bool:
-        """Check if Redis is reachable."""
-        if not self.client:
-            return False
-        try:
-            await self.client.ping()
-            return True
-        except redis.RedisError:
-            return False
-
-
-# Global instances
+# Global instance
 db = Database()
-redis_client = RedisClient()
 
 
 _connection_ctx: ContextVar[asyncpg.Connection | PoolConnectionProxy | None] = ContextVar(