docs: describe the design the code has, not the two before it

Three reversals had landed without the documentation following them, so the
docs described a pipeline that compresses its output, a schema with three
secondary indexes, and a browser that re-downloads the file on every visit.
None of those are true any more.

- Compression: the assembler stopped producing .gz when the databases began
  shipping as .sqlite3. The deployment guide's "why no uncompressed database
  can ship" section explained a guard that now exists for the opposite reason
  — to keep .db, .gz and journals out, so .sqlite3 stays the only name.
- Indexes: the architecture printed a DDL with three CREATE INDEX statements
  and a paragraph on the partial one. schema.go carries none.
- Persistence: "the download is repeated every visit ... has not been done"
  was listed as an open risk after db-cache.js closed it. Replaced with the
  ETag flow, the offline fallback, and the risks that did replace it.

Measured both transfers rather than scaling one from the other, which would
have been wrong: 2016 is 142 MB stored and 31 MB delivered, 2017 is 119 MB and
36 MB. The smaller database is the larger download, so neither figure follows
from the stored size.

Also corrects a CHUNK_BYTES reference to a module that no longer exists, the
238-289 MB per-dataset figure, two paths to web/src/lib/datasets.js, and the
CI step list, which omitted npm test and the post-deploy header check.

The two code comments that said the same outdated things go with them.

assembler/internal/databases/databases.go

@@ -73,10 +73,12 @@ func BuildParser(p Paths) (string, error) {
 	return bin, nil
 }
 
-// Build runs the parser for one dataset, verifies the result and compresses it.
+// Build runs the parser for one dataset and verifies the result.
 //
-// Only the .gz survives: shipping a 100+ MB uncompressed database is made
-// structurally impossible rather than left to a cleanup step.
+// Two guards, both refusing to publish rather than warning: the row count must
+// equal the registry's exactly, and the file must be at least minSizeRatio of
+// the size the registry records. A truncated or short database is the failure
+// that would otherwise reach the site with a green pipeline.
 func Build(p Paths, bin string, d registry.Dataset) error {
 	if err := os.MkdirAll(p.OutDir, 0o755); err != nil {
 		return err