Alloy V1.16.0 (#345)

Bump Grafana Alloy v1.14.0 → v1.16.0 across all four services (alloy-k8s, alloy-ringtail, alloy-tracing-ringtail; alloy native ansible). Also migrate the indri build path from `Dockerfile` to a native Dagger `container.py` per the build-container-image migration playbook.

## Highlights from upstream
- v1.15: database observability promoted to stable, OTel Collector → v0.147.0
- v1.16: clustering for `loki.source.kubernetes_events`, MySQL exporter 0.19.0
- One pre-existing breaking change in v1.15 (`loki.source.awsfirehose` undocumented metric prefix rename) — not used here.

## Build infra
Alloy v1.16.0's go.mod requires Go 1.26.2. The nix derivation now uses `pkgs.go_1_26` with `GOTOOLCHAIN=local` to avoid auto-downloading a toolchain blob that violated the fixed-output rule.

## Test plan
- [ ] CI: `mise run container-build-and-release alloy --ref alloy-v1.16.0` (dispatched as run 522; nix job to be re-triggered with the v1.16.0 goModules outputHash once the local ringtail build surfaces it)
- [ ] After CI green, bump `images[].newTag` in three kustomizations to the new `-<sha>` and `-<sha>-nix` tags, deploy from this branch via `argocd app set <app> --revision alloy-v1.16.0 && argocd app sync <app>`
- [ ] Manual rebuild of macOS native binary on gilbert (per ansible/roles/alloy README) and `mise run provision-indri -- --tags alloy --check --diff`
- [ ] `mise run services-check` after merge & redeploy

Reviewed-on: #345

docs/changelog.d/alloy-v1.16.0.infra.md

@@ -0,0 +1,5 @@
+Upgrade Grafana Alloy v1.14.0 → v1.16.0 across all four service deployments
+(alloy-k8s, alloy-ringtail, alloy-tracing-ringtail on k8s; alloy native on
+indri). Pulls in stable database observability (v1.15) and the OTel Collector
+v0.147.0 bump. Container build also migrated from Dockerfile to native Dagger
+`container.py` per the build-container-image migration playbook.