Make the README's storage, formats, TLS and metrics match the code (closes #74)

"Storage" names the cache directories pixa uses (cache/sources,
cache/metadata, cache/variants) and how files are named in each; the
schema comments name the same paths, and the output_content comment says
the table is not written. Routes and the signature section give one list
of output formats, jpg and original included, and say how those two are
signed. The TLS sentence names allow_http as its exception. "Metrics"
says what exists: generic HTTP and Go runtime metrics at /metrics,
measured and served only when the metrics username and password are set.

Model: opus-5-5

TODO.md

@@ -29,6 +29,14 @@ P2: security: referer blacklist
 
 # Completed Steps
 
+- 2026-10-04 `README.md` matches the code (closes #74): "Storage" names the
+  cache directories pixa uses (`cache/sources`, `cache/metadata`,
+  `cache/variants`) and how files are named in each, and the comments in
+  `001_schema.sql` name the same paths; the routes and the signature section
+  list the same output formats, `jpg` and `original` included; the TLS
+  sentence names `allow_http` as its exception; "Metrics" says only generic
+  HTTP and Go runtime metrics exist, measured and served only when the metrics
+  username and password are set.
 - 2026-10-03 shutdown sets the exit code and waits for image processing
   (closes #86): fx alone handles SIGINT and SIGTERM, and the server's own
   signal handler is gone; fx's `Run` in `cmd/pixad` exits with the shutdown's