feat(blog): taxonomies and client-side filter UI (#499)

## Summary

Adds two blog-specific taxonomies (`article_types`, `topics`) and a
client-side filter UI on the blog index so readers can narrow `/blog/`
by article type and/or subject area.

## What

**Data model — `hugo.yaml`**
- Registers two new taxonomies: `article_type` / `article_types` and
`topic` / `topics`.
- Preserves Hugo's built-in `tag` and `category` taxonomies so nothing
breaks if they are used later.

**Tagging — every existing post**
- 68 posts tagged across `content/en/blog/` with a combination of:
- `article_types`: `release | how-to | case | tech-article | news`
(usually one).
- `topics`: `platform, postgresql, kubernetes, cilium, talos,
opensearch, kubevirt, networking, storage, security, events, community,
gpu` (zero to three, only when the subject is a main theme of the post —
not incidental mentions).
- Release posts get `platform` by default, plus any headline features
that were clearly the reason for the release (e.g. OpenSearch, GPU
passthrough).

**Filter UI — `layouts/blog/list.html`**
- Overrides the Docsy blog list template. Two rows of chips above the
post list: **Type** (fixed order: release → how-to → case → tech-article
→ news) and **Topic** (auto-sorted).
- Click a chip to toggle. Multiple chips within one dimension combine
with OR; the two dimensions combine with AND.
- State is mirrored into the URL:
`/blog/?types=how-to&topics=postgresql,kubernetes`. Shareable,
back/forward works.
- Live counter: "Showing X of Y posts". Year groups collapse when empty
under the current filter.
- Pagination is dropped — for ~70 posts, showing the full archive once
keeps filtering honest (otherwise filters only affect the visible page).

## Why

Until now, the blog has been strictly chronological with no way to
narrow down. Readers coming in from a community share ("how-to posts
only", "anything about OpenSearch", "Cozystack release history") had to
scroll the full archive.

This change takes advantage of existing structure — most posts already
fall cleanly into one of the five editorial types and a small set of
topics — without introducing new maintenance burden: adding tags to new
posts is a two-line frontmatter change.

## Preview

Netlify deploy preview will render the full filter UI on `/blog/`.
Things worth checking:

- Chip layout on mobile (should wrap gracefully).
- Empty-state when a filter combination matches zero posts.
- URL deep-linking: open `/blog/?types=release&topics=platform` directly
and the chips should arrive pre-selected.
- Dark-mode contrast on chips (uses Bootstrap CSS variables).

## Follow-ups (not in this PR)

- Make post tags on each card clickable so they act as filter shortcuts
(currently they go to the Hugo-generated taxonomy list pages at
`/article-types/<slug>/` and `/topics/<slug>/`, which still work but are
separate URLs).
- Optional: server-rendered filter state for SEO on common filter
combinations.

Author: tym83

content/en/blog/2020-05-14-configuring-routing-for-metallb-in-l2-mode.md

@@ -6,6 +6,10 @@ author: "Andrei Kvapil"
 description: "In this article I will show you how to configure source-based and policy-based routing for the external network on your cluster."
 images:
   - "https://cdn-images-1.medium.com/max/800/0*wI1GLh4MrCzuwiwB.png"
+article_types:
+  - how-to
+topics:
+  - networking
 
 ---
 

content/en/blog/2024-02-20-cozystack-v0-1.md

@@ -4,6 +4,12 @@ slug: cozystack-v0-1
 date: 2024-02-20
 author: "Timur Tukaev"
 description: "The first feature release of Cozystack introduces ZFS storage support, leader election for the controller, and moves documentation to cozystack.io."
+article_types:
+  - release
+topics:
+  - platform
+  - storage
+
 ---
 
 ### Cozystack v0.1: ZFS Support, Leader Election, and Documentation Move

content/en/blog/2024-02-21-introducing-cozystack-a-free-paas-platform-based-on-kubernetes.md

@@ -4,6 +4,10 @@ slug: introducing-cozystack-a-free-paas-platform-based-on-kubernetes
 date: 2024-02-21
 images:
   - "https://cozystack.io/img/screenshot.png"
+article_types:
+  - news
+topics:
+  - platform
 
 ---
 

content/en/blog/2024-04-05-cozystack-v0-2.md

@@ -4,6 +4,11 @@ slug: cozystack-v0-2
 date: 2024-04-05
 author: "Timur Tukaev"
 description: "Cozystack v0.2 introduces bundle-based installation, schema versioning for charts, moves FluxCD to the core, and updates Cilium, LINSTOR, CNPG, and MariaDB Operator."
+article_types:
+  - release
+topics:
+  - platform
+
 ---
 
 ### Cozystack v0.2: Bundles, Schema Versioning, FluxCD as Core Package, and Component Updates

content/en/blog/2024-04-05-diy-create-your-own-cloud-with-kubernetes-part-1/index.md

@@ -2,6 +2,12 @@
 title: "DIY: Create Your Own Cloud with Kubernetes (Part 1)"
 slug: diy-create-your-own-cloud-with-kubernetes-part-1
 date: 2024-04-05T07:30:00+00:00
+article_types:
+  - tech-article
+topics:
+  - platform
+  - kubernetes
+
 ---
 
 **Author**: Andrei Kvapil (Ænix)

content/en/blog/2024-04-05-diy-create-your-own-cloud-with-kubernetes-part-2/index.md

@@ -2,6 +2,14 @@
 title: "DIY: Create Your Own Cloud with Kubernetes (Part 2)"
 slug: diy-create-your-own-cloud-with-kubernetes-part-2
 date: 2024-04-05T07:35:00+00:00
+article_types:
+  - tech-article
+topics:
+  - platform
+  - kubernetes
+  - networking
+  - storage
+
 ---
 
 **Author**: Andrei Kvapil (Ænix)

content/en/blog/2024-04-05-diy-create-your-own-cloud-with-kubernetes-part-3/index.md

@@ -2,6 +2,12 @@
 title: "DIY: Create Your Own Cloud with Kubernetes (Part 3)"
 slug: diy-create-your-own-cloud-with-kubernetes-part-3
 date: 2024-04-05T07:40:00+00:00
+article_types:
+  - tech-article
+topics:
+  - platform
+  - kubernetes
+
 ---
 
 **Author**: Andrei Kvapil (Ænix)

content/en/blog/2024-04-18-cozystack-v0-3.md

@@ -4,6 +4,11 @@ slug: cozystack-v0-3
 date: 2024-04-18
 author: "Timur Tukaev"
 description: "Cozystack v0.3 adds Kafka and ClickHouse as managed applications and introduces NoCloud assets for Hetzner bare-metal installation."
+article_types:
+  - release
+topics:
+  - platform
+
 ---
 
 ### Cozystack v0.3: Kafka, ClickHouse, and Hetzner Bare-Metal Support

content/en/blog/2024-05-06-cozystack-v0-4.md

@@ -4,6 +4,11 @@ slug: cozystack-v0-4
 date: 2024-05-06
 author: "Timur Tukaev"
 description: "Cozystack v0.4 replaces kamaji-etcd with the new aenix-io/etcd-operator, adds replica configuration for applications, and updates Kamaji, LINSTOR, and Kubeapps."
+article_types:
+  - release
+topics:
+  - platform
+
 ---
 
 ### Cozystack v0.4: etcd Operator, Replica Options, Kamaji v0.5, and Dark Mode Fix

content/en/blog/2024-05-10-cozystack-v0-5.md

@@ -4,6 +4,12 @@ slug: cozystack-v0-5
 date: 2024-05-10
 author: "Timur Tukaev"
 description: "Cozystack v0.5 adds automatic Helm schema generation, removes hardcoded defaults, and updates Cilium and MariaDB Operator."
+article_types:
+  - release
+topics:
+  - platform
+  - cilium
+
 ---
 
 ### Cozystack v0.5: Automatic Schema Generation, Cilium v1.14.10, and MariaDB Operator Update