docs: add PROGRESS.md with full shipping history and production metrics

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

PROGRESS.md

@@ -0,0 +1,75 @@
+# MSDL — Progress Log
+
+## Shipped
+
+### eval ISOs — PR #13 `feat/evalcenter-isos`
+**Windows Server 2016–2025 + Windows 11 Enterprise evaluation editions**
+
+- `GET /evallinks?product=<slug>` endpoint — resolves Microsoft Eval Center fwlink redirects, caches for 24h, warms on startup
+- Frontend eval section: `/eval` listing page + `/eval/:slug` detail page with download links
+- Eval products: `server-2025`, `server-2022`, `server-2019`, `server-2016`, `win11-ent`
+- ComparisonTable + FAQAccordion updated to mention eval ISOs
+- HowItWorks step 1 updated
+- SEO audit across all pages (meta descriptions, robots noindex on legal pages, JSON-LD on product detail)
+- README: `/evallinks` API docs, eval product table, contributing guide for eval editions
+
+---
+
+### Two-layer caching — PR #14 + #15 `feat/caching-layer` (closes #12)
+**Reduced Microsoft API calls from thousands/day to ~50–100/day**
+
+- **Singleflight** (`golang.org/x/sync/singleflight`) — collapses concurrent cache misses into 1 Microsoft fetch
+- **SKU cache** — 7-day TTL, keyed by `product_id`
+- **Link cache** — dynamic TTL parsed from `se` param in signed Microsoft CDN URL, minus 30min buffer
+- **Eval cache** — 24h TTL, keyed by slug, warmed at startup
+- **Negative cache** — 60s TTL, prevents thundering-herd retries during rate-limit blocks
+- **Stale-on-failure** — serves expired entry if refresh fails; background refresh via singleflight
+- **Jitter** — ±5min random offset on TTLs to prevent synchronized mass-expiry
+- **Cache eviction** — background goroutine every 30min, logs only when entries deleted
+- **Cache hit/miss logging** — every request logs fetched vs cached + expiry timestamp
+- Dockerfile bumped to `golang:1.25-alpine` to match `go.mod`
+
+---
+
+### Metrics endpoint — `feat/metrics`
+**Real-time cache observability**
+
+- `GET /metrics?secret=<secret>` — returns cache hit rates, miss counts, MS fetch totals, cache sizes
+- Auth via `METRICS_SECRET` env var (`?secret=` param or `Authorization: Bearer` header)
+- Atomic counters (`sync/atomic`) — lock-free, resets on restart
+- Covers SKU, link, eval, and negative caches
+- Background stale refresh routed through singleflight (prevents race condition under concurrent stale hits)
+
+---
+
+### README architecture docs — direct to `main`
+
+- How It Works flow diagram updated with cache check step
+- Caching layer section — table of all 4 caches, singleflight, stale-on-failure, jitter, eviction
+- Backend Tech Stack table updated (Go 1.25+, real cache description)
+- `/metrics` added to API Reference with example response
+- `METRICS_SECRET` added to env vars block
+
+---
+
+## Production metrics snapshot (after ~1 day of traffic)
+
+```json
+{
+  "sku":  { "requests": 37, "cache_hits": 29, "ms_fetches": 8,  "hit_rate": "78.4%" },
+  "link": { "requests": 28, "cache_hits": 14, "ms_fetches": 14, "hit_rate": "50.0%" },
+  "eval": { "requests": 2,  "cache_hits": 1,  "stale": 0,       "hit_rate": "50.0%" },
+  "total_ms_fetches": 22
+}
+```
+67 user requests → 22 Microsoft calls. **67% of calls eliminated** on a still-warming cache.
+CF Worker ensures Hetzner IP is never exposed to Microsoft — rate-limit block (715-123130) is effectively impossible.
+
+---
+
+## Deferred
+
+| Item | Notes |
+|---|---|
+| Per-IP / per-product rate limiter | Revisit after 1 month of production traffic data |
+| `_redirects` Cloudflare Pages bug | SPA fallback rule flagged as infinite loop, may cause 404s on direct nav |