feat: rewrite ClamAV exporter in Go

[tharinfernando]

Description

Go port of clamav-exporter/exporter.py with identical metrics and endpoints.

• Closes [TASK] Write ClamAV exporter in Go. OpenGovMail#291

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Changes Made

• Add clamav-exporter/main.go: stdlib HTTP server, Unix socket probe with TCP clamav:3310 fallback, z<CMD> null-terminated protocol with 5s timeout, STATS/VERSION parsing, clamd and freshclam log parsing, Prometheus exposition for all 13 metrics
• Add clamav-exporter/main_test.go: 9 unit tests (STATS, VERSION, log parsing, freshclam status, golden exposition output, down-state)
• Add clamav-exporter/go.mod (go 1.21, stdlib only), Makefile (build, test, vet, docker-build), README.md (build/run docs, env vars, memory comparison)
• Replace clamav-exporter/Dockerfile Python base with multi-stage Go build on alpine, non-root user, same EXPOSE 9810
• Keep exporter.py in place until the Go version is verified in deployment

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Checklist (Email System)

• Core services tested (SMTP, IMAP, mail storage, end-to-end delivery) - N/A, exporter only, no mail path change
• Security and compliance verified (auth via Thunder IDP, TLS, DKIM/SPF/DMARC, spam/virus filtering) - N/A, metrics only; Dockerfile runs as non-root appuser
• Configuration and deployment checked (configs generated, Docker/Compose updated) - same build context and port, compose unchanged
• Reliability confirmed (error handling, logging, monitoring) - fail-soft clamav_up 0 when ClamAV unreachable, sanitized logs, 5s timeouts
• Documentation and usage notes updated (README, deployment, API) - README with build, run, env, and memory numbers

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Testing Instructions

1. cd clamav-exporter && go test ./... && go vet ./...
2. STARTUP_DELAY_SECONDS=0 go run main.go, then curl -s localhost:9810/health returns OK
3. Against a stub ClamAV TCP server replying canned STATS and VERSION: curl -s localhost:9810/metrics shows clamav_up 1 and all 13 metrics with correct values
4. docker build -t opengovmail-clamav-exporter . succeeds, image 27.1MB vs python:3.11-slim base 198MB

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Screenshots / Logs

 clamav_up 1
 clamav_state 1
 clamav_pools_total 2
 clamav_threads_live 10
 clamav_threads_idle 8
 clamav_queue_items 3
 clamav_memory_heap_mb 100.5
 clamav_memory_used_mb 80.25
 clamav_signature_version 26853
 clamav_version{version="1.0.0"} 1
 clamav_viruses_found_total 0
 clamav_files_scanned_total 0
 clamav_database_updated 0

Memory (host RSS, no backend reachable): Python 23,512 KB idle and after 50 scrapes; Go 10,084 KB idle and 10,912 KB after 50 scrapes.

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Notes for Reviewers

• One intentional behavior difference: Python always reports clamav_up 1 because log stats are merged into the stats map before the truthiness check, so it never detects a down daemon. The Go version reports clamav_up 0 when STATS and VERSION both fail. Flagging for approval or I can match the old behavior.
• exporter.py deletion is deferred to a follow-up after this is verified live, per the issue.
• No compose or prometheus config changes needed: same directory, same port 9810, same scrape target.