Fix frontmatter field name for Quartz date display (#158)

## Summary

- Rename `date-modified` -> `modified` in all 80 docs and the `docs-check-frontmatter` task

Quartz's `CreatedModifiedDate` plugin recognizes `modified`, `lastmod`, `updated`, and `last-modified` — but not `date-modified`. The wrong field name caused Quartz to ignore frontmatter dates entirely and fall through to filesystem timestamps (UTC inside Dagger), showing Feb 12 on pages built late on Feb 11 PST.

## Test plan

- [x] `mise run docs-check-frontmatter` passes
- [ ] Kick off docs release after merge — verify rendered dates match frontmatter values

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/158

docs/reference/infrastructure/gandi.md

@@ -1,6 +1,6 @@
 ---
 title: Gandi
-date-modified: 2026-02-08
+modified: 2026-02-08
 tags:
   - infrastructure
   - networking

docs/reference/infrastructure/gilbert.md

@@ -1,6 +1,6 @@
 ---
 title: Gilbert
-date-modified: 2026-02-07
+modified: 2026-02-07
 tags:
   - infrastructure
   - host

docs/reference/infrastructure/hosts.md

@@ -1,6 +1,6 @@
 ---
 title: Hosts
-date-modified: 2026-02-10
+modified: 2026-02-10
 tags:
   - infrastructure
 ---

docs/reference/infrastructure/indri.md

@@ -1,6 +1,6 @@
 ---
 title: Indri
-date-modified: 2026-02-09
+modified: 2026-02-09
 tags:
   - infrastructure
   - host

docs/reference/infrastructure/power.md

@@ -1,6 +1,6 @@
 ---
 title: Power
-date-modified: 2026-02-09
+modified: 2026-02-09
 tags:
   - infrastructure
 ---

docs/reference/infrastructure/routing.md

@@ -1,6 +1,6 @@
 ---
 title: Routing
-date-modified: 2026-02-09
+modified: 2026-02-09
 tags:
   - infrastructure
   - networking

docs/reference/infrastructure/tailscale.md

@@ -1,6 +1,6 @@
 ---
 title: Tailscale
-date-modified: 2026-02-08
+modified: 2026-02-08
 tags:
   - infrastructure
   - networking

docs/reference/infrastructure/unifi.md

@@ -1,6 +1,6 @@
 ---
 title: UniFi
-date-modified: 2026-02-10
+modified: 2026-02-10
 tags:
   - infrastructure
   - networking