refactor: split into web/crawler/go-parser and drop the 2017 archives

Move the frontend into web/, the repo's only npm workspace, and replace the
JS crawler with a Go module covering both remaining datasets. The crawler
writes to a .part file and renames on completion: writing straight to the
destination left truncated files that the skip-if-present check would then
skip forever.

Remove the 2017-old and 2017-old2 datasets. They were successive publications
of the same exam, kept side by side so the disagreement stayed inspectable;
the current 2017 supersedes them and they remain in git history.

Recover the 2016 crawler source from the Internet Archive's copy of the
aggregator article, whose original host no longer resolves. All 119 filenames
are verified against data/2016 in both directions, but no archive captured the
spreadsheets themselves, so the host still serving them is unconfirmed and
data/2016 remains the only confirmed copy.

Filenames are load-bearing throughout: go-parser sorts inputs bytewise and
inserts last-wins, so they decide which row survives a duplicate exam number.

docs/deployment-guide.md

@@ -1,7 +1,7 @@
 # Deployment Guide
 
 Deploys to GitHub Pages via `.github/workflows/deploy-pages.yml`. Every push to
-`main` rebuilds all four datasets and redeploys the whole site.
+`main` rebuilds both datasets and redeploys the whole site.
 
 One-time setup: **Settings → Pages → Source: GitHub Actions**.
 
@@ -9,9 +9,9 @@ One-time setup: **Settings → Pages → Source: GitHub Actions**.
 
 1. Checkout, Go toolchain, Node 24, `npm ci`
 2. `npm run build:go` — one parser binary
-3. `npm run build:db` — builds and gzips all four databases into
+3. `npm run build:db` — builds and gzips both databases into
    `.build/public/db/`
-4. `npm run build:site` — one Vite build, then `scripts/assemble-site.js`
+4. `npm run build:site` — one Vite build, then `web/scripts/assemble-site.js`
 5. `actions/upload-pages-artifact` + `actions/deploy-pages`
 
 The database build dominates the runtime: roughly 419 MB of Excel is parsed on
@@ -23,20 +23,18 @@ every deploy.
 https://<user>.github.io/thptqg/
 https://<user>.github.io/thptqg/2016/
 https://<user>.github.io/thptqg/2017/
-https://<user>.github.io/thptqg/2017-old/
-https://<user>.github.io/thptqg/2017-old2/
 ```
 
-`/thptqg/2017/old/` and `/thptqg/2017/old2/` were the pre-flattening URLs. They
-are still served, and the router rewrites them to the flat form with the query
-string intact.
+`/thptqg/2017/old/` and `/thptqg/2017/old2/` were the pre-flattening URLs for
+the two removed 2017 archives. They are no longer served; like any unknown path
+they now render the hub via `404.html`.
 
 ## Local reproduction
 
 ```bash
 npm ci
 npm run build:go
-npm run build:db      # all four; pass an id to build just one
+npm run build:db      # both; pass an id to build just one
 npm run build:site    # vite build + assemble into _site/
 npx serve _site
 ```
@@ -44,7 +42,7 @@ npx serve _site
 To rebuild a single dataset:
 
 ```bash
-node go-parser/scripts/build-db.js 2017-old
+npm run build:db 2017
 ```
 
 ## Base path
@@ -59,7 +57,7 @@ up as a blank page with 404s on `/assets/...`.
 2. Add `go-parser/configs/<id>.yml` with the parse rules — sheet mode, column
    indices, SBD validation, header tokens, blank-row stripping. No SQL: the
    schema is canonical and lives in `go-parser/internal/schema/schema.go`
-3. Add an entry to `DATASETS` in `src/datasets.js`
+3. Add an entry to `DATASETS` in `web/src/datasets.js`
 
 Nothing else. The build script, the site assembly and the router all read that
 one list, and the frontend adapts to whichever columns the dataset populates.
@@ -98,7 +96,7 @@ run rebuilds the older state. There is no data to migrate.
 | Symptom | Typical cause |
 | --- | --- |
 | Blank page, 404 on assets | `base` in `vite.config.js` does not match the repo name |
-| `Failed to fetch database: 404` | Dataset id in `src/datasets.js` does not match the file in `db/` |
+| `Failed to fetch database: 404` | Dataset id in `web/src/datasets.js` does not match the file in `db/` |
 | A route 404s | `assemble-site.js` did not run, or the id is missing from `DATASETS` |
 | WASM fails to load | `sql.js.org` unreachable — self-host `sql-wasm.wasm` and update `SQL_WASM_URL` in `use-sqlite.js` |
 | Deploy fails on assembly | An uncompressed database artefact reached the output; the error names the files |