Add cost service for per-VIN cost estimation

services/cost/README.md

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+# Cost Service
+
+Per-vehicle cost estimation service for capacity planning and cloud vs on-prem comparison.
+
+## Overview
+
+This service estimates the cost of running cloud services per VIN by:
+1. Querying vehicle activity from ClickHouse (message counts)
+2. Estimating resource usage based on activity level
+3. Applying cost rates for cloud vs on-prem hosting
+4. Storing aggregated cost data for reporting
+
+## Cost Estimation Methodology
+
+### Resource Estimates Per Active VIN
+
+| Activity Level | Messages/15min | CPU (cores) | Memory (GB) |
+|---------------|----------------|-------------|-------------|
+| Low           | < 100          | 0.05        | 0.1         |
+| Medium        | 100-1000       | 0.075       | 0.15        |
+| High          | > 1000         | 0.10        | 0.2         |
+
+These are **estimates** based on typical workload patterns, not actual measurements.
+
+### Cost Rates (per hour)
+
+| Resource | Cloud (Azure) | On-Prem |
+|----------|---------------|---------|
+| CPU/core | $0.08         | $0.02   |
+| Memory/GB| $0.015        | $0.004  |
+
+#### Cloud Rates (Fudged Higher)
+- Based on Azure D-series VM pricing + 20% overhead
+- Includes: compute, managed services, networking, support
+- Intentionally conservative (higher) to show cloud costs
+
+#### On-Prem Rates (Fudged Lower)
+- Based on 3-year hardware amortization
+- Assumes: owned hardware, minimal ops overhead
+- Intentionally optimistic (lower) to show on-prem savings
+- Does NOT include: datacenter costs, staff, power, cooling
+
+### Savings Calculation
+
+```
+Cloud Cost = (CPU_cores × $0.08 + Memory_GB × $0.015) × hours
+On-Prem Cost = (CPU_cores × $0.02 + Memory_GB × $0.004) × hours
+Savings = Cloud Cost - On-Prem Cost
+Savings % = (Savings / Cloud Cost) × 100
+```
+
+Expected savings: **~70-75%** with on-prem hosting.
+
+## API Endpoints
+
+### GET /cost/vin/{vin}
+Cost summary for a specific VIN.
+
+### GET /cost/fleet
+Fleet-wide cost summary with top cost VINs.
+
+### GET /cost/summary?period=day|week|month
+High-level cost summary for a time period.
+
+### GET /cost/comparison
+Cloud vs on-prem cost comparison with projected annual savings.
+
+## Configuration
+
+| Env Var | Description | Default |
+|---------|-------------|---------|
+| CLICKHOUSE_HOST | Local CH for storing cost data | localhost |
+| REMOTE_CLICKHOUSE_HOST | Dev cluster CH for VIN activity | - |
+| COLLECTOR_INTERVAL_MINUTES | How often to collect metrics | 15 |
+
+## Limitations
+
+- Resource estimates are approximations, not actual measurements
+- Cost rates are simplified and don't reflect all real-world factors
+- On-prem costs exclude significant operational overhead
+- Designed for business case illustration, not precise billing