chore: restructure to per-stack monorepo layout

[vpatrin]

Summary

• Restructure services/ → stacks/ with one docker-compose.yml per stack (postgres, caddy, coupette-redis, umami, uptime-kuma, observability)
• Rewrite scripts/deploy_infra.sh to iterate stacks in dependency order and discover *.env.enc dynamically under stacks/
• Update .sops.yaml regex, .github/dependabot.yml directories, and CI compose validation to match the new layout
• Add ref input to .github/workflows/deploy.yml + make deploy REF=... so any branch matching main|chore/*|feat/*|fix/*|docs/* can be deployed end-to-end before merging
• Update slash command paths (.claude/commands/*.md) and README tree to match the new layout

Changes

• stacks/ — per-stack composes; service definitions, networks, volumes, container names unchanged from previous root compose
• scripts/deploy_infra.sh — loops over stacks, dynamic SOPS discovery, per-stack validate → pull → up, Caddy validate + reload, systemd unit sync, 7 retried healthchecks
• Makefile — targets refactored to iterate $(STACKS); deploy now accepts REF=...
• .github/workflows/deploy.yml — new ref input, git fetch && git checkout && git pull --ff-only on the VPS
• .sops.yaml — services/.+/\.env(\.enc)?$ → stacks/.+/\.env(\.enc)?$
• .github/workflows/ci.yml — compose job now validates each stack including dev/prod overrides
• .github/dependabot.yml — directories list updated to per-stack paths

Deploy notes

• Production environment branch protection updated via gh api to allow main, chore/*, feat/*, fix/*, docs/* patterns (was main only)
• After squash-merge, redeploy with make deploy (defaults to main) — the VPS is currently checked out on this branch from pre-merge testing

Test plan

• All 6 stack composes validate locally including prod overrides
• Caddyfile validates against caddy:2.11.2
• make deploy REF=chore/monorepo-restructure — full prod deploy from branch, all healthchecks green
• No stale services/ references in any executable surface (composes, scripts, systemd, CI, Makefile, slash commands)