perf(images): WebP banners, text-only related cards, resized list thumbnails

Measured before/after with headless Chrome + CDP (not by eye):

- article banner: <picture> WebP q78 with the PNG as fallback, because the PNG
  must stay for og:image (social scrapers do not reliably render WebP cards).
  70 banners: 6.02 MB -> 2.52 MB (42%). JPEG measured *larger* than PNG here
  (q85 = 129%) — the art is flat terminal-style with sharp text edges.
- post page related cards: dropped four 1200x630 PNGs (183 KB) to display
  526x276 where the embedded code is illegible; now text-only cards with a
  category badge, date and reading time.
- list thumbnails (home + archive): new /og/thumb/<name>-{400,800}.webp served
  via srcset/sizes. Rows display at ~398px but were pulling full 1200x630 PNGs.
  Homepage thumbnails ~450 KB -> 62 KB; archive (71 rows) ~3.2 MB -> 436 KB.
- desktop banner capped at 720px centred: 67% -> 45% of a 900px viewport,
  keeping the art intact instead of cropping it.

Post page totals: images 277 KB -> 38 KB; page height 5758 -> 5065 (desktop)
and 9423 -> 8807 (mobile). Build: 163 pages.

Script: scripts/optimize-images.py; policy: docs/design-guide.md §3; runbook
step added to docs/ops-runbook.md.

scripts/optimize-images.py

@@ -0,0 +1,74 @@
+#!/usr/bin/env python3
+"""Generate the derivative images the site serves in-page.
+
+Three outputs, all derived from the existing PNGs:
+
+1. `public/banners/<slug>.webp`      article banner, q78  (~42% of the PNG)
+2. `public/og/thumb/<name>-800.webp` list thumbnail @2x, q72
+3. `public/og/thumb/<name>-400.webp` list thumbnail @1x, q72
+
+Why not JPEG anywhere: this art is flat terminal-style with sharp text edges, and it
+measured *larger* as JPEG q85 than as PNG (129%); WebP came out at 42%. The PNGs stay
+the source of truth for `og:image` (social scrapers do not reliably render WebP share
+cards), and the banner PNG doubles as the <picture> fallback.
+
+Usage:
+    uv run --with pillow python scripts/optimize-images.py             # everything
+    uv run --with pillow python scripts/optimize-images.py <slug> ...  # one post
+"""
+import glob, os, sys
+from PIL import Image
+
+ROOT = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
+BANNERS = os.path.join(ROOT, "public", "banners")
+OG = os.path.join(ROOT, "public", "og")
+THUMBS = os.path.join(OG, "thumb")
+BANNER_Q = 78
+THUMB_Q = 72
+
+def save_webp(im, path, quality):
+    os.makedirs(os.path.dirname(path), exist_ok=True)
+    if im.mode not in ("RGB", "RGBA"):
+        im = im.convert("RGBA" if "A" in im.mode else "RGB")
+    im.save(path, "WEBP", quality=quality, method=6)
+    return os.path.getsize(path)
+
+def banner(slug):
+    png = os.path.join(BANNERS, slug + ".png")
+    if not os.path.exists(png):
+        return None
+    return os.path.getsize(png), save_webp(Image.open(png), png[:-4] + ".webp", BANNER_Q)
+
+def thumbs(slug):
+    png = os.path.join(OG, slug + ".png")
+    if not os.path.exists(png):
+        return None
+    im = Image.open(png)
+    src = os.path.getsize(png)
+    out = []
+    for w in (400, 800):
+        h = round(w * 630 / 1200)
+        out.append(save_webp(im.resize((w, h), Image.LANCZOS),
+                             os.path.join(THUMBS, f"{slug}-{w}.webp"), THUMB_Q))
+    return src, out
+
+targets = sys.argv[1:]
+slugs = targets or sorted(os.path.basename(p)[:-4] for p in glob.glob(os.path.join(OG, "*.png"))
+                          if not p.endswith("og-default.png"))
+
+b_png = b_webp = 0
+t_png = t_400 = t_800 = 0
+for s in slugs:
+    b = banner(s)
+    if b:
+        b_png += b[0]; b_webp += b[1]
+    t = thumbs(s)
+    if t:
+        t_png += t[0]; t_400 += t[1][0]; t_800 += t[1][1]
+
+n = max(1, len(slugs))
+print(f"banners : {len(slugs)} files  {b_png/1024/1024:.2f} MB PNG -> {b_webp/1024/1024:.2f} MB WebP "
+      f"({100*b_webp/max(1,b_png):.0f}%)   [{b_webp/n/1024:.1f} KB/file]")
+print(f"thumbs  : 400w {t_400/1024:.0f} KB ({t_400/n/1024:.1f} KB/file) + 800w "
+      f"{t_800/1024:.0f} KB ({t_800/n/1024:.1f} KB/file)  vs PNG {t_png/1024/1024:.2f} MB "
+      f"({100*(t_400+t_800)/max(1,t_png):.0f}% of PNG)")