Fix Lighthouse audit findings: contrast, cache, robots.txt

- Card link color bumped to accent-high for WCAG AA contrast ratio
- Caddyfile: immutable cache headers for hashed _astro/* assets
- robots.txt: move LLMs-Txt to comments (not a standard directive)

docs/Caddyfile.prod

@@ -1,6 +1,14 @@
 :4321 {
 	encode gzip
 
+	# Hashed assets — immutable, cache forever
+	@hashed path /_astro/*
+	header @hashed Cache-Control "public, max-age=31536000, immutable"
+
+	# OG images — content-hashed filenames aren't used, cache 1 day
+	@og path /og/*
+	header @og Cache-Control "public, max-age=86400"
+
 	handle {
 		root * /usr/share/caddy
 		try_files {path} {path}/index.html

docs/public/robots.txt

@@ -3,5 +3,7 @@ Allow: /
 
 Sitemap: https://mcnoaa-tides.warehack.ing/sitemap-index.xml
 
-# LLM documentation — https://llmstxt.org/
+# LLM-readable documentation: https://llmstxt.org/
-LLMs-Txt: https://mcnoaa-tides.warehack.ing/llms.txt
+# Full docs:     https://mcnoaa-tides.warehack.ing/llms-full.txt
+# Abridged docs: https://mcnoaa-tides.warehack.ing/llms-small.txt
+# Overview:      https://mcnoaa-tides.warehack.ing/llms.txt

docs/src/styles/custom.css

@@ -177,13 +177,13 @@ nav.sidebar .top-level > li > a {
   border: none !important;
   font-size: 0.85rem;
   font-weight: 600;
-  color: var(--sl-color-accent) !important;
+  color: var(--sl-color-accent-high) !important;
   text-decoration: none;
   transition: color 0.15s ease;
 }
 
 [data-has-hero] .sl-markdown-content article.card a[href]:hover {
-  color: var(--sl-color-accent-high) !important;
+  color: var(--sl-color-white) !important;
 }
 
 /* Install section: center the install block */