feat(web): add --auth-file so credentials don't pass through argv

[TbusOS]

Implements option (1) from #10 — --auth-file.

What changed

cli/engram/commands/web.py:

• --auth-file PATH — reads USER:PASS from the first non-empty line. The
  secret sits behind filesystem permissions instead of being published through
  argv.
• Permission warning — if the file is group- or other-readable, serve prints
  a warning naming the mode and the chmod 600 fix. A world-readable credential
  file would give back exactly what this route was meant to gain, so it should not
  fail silently.
• Mutual exclusion — passing both --auth and --auth-file is an error rather
  than a silent precedence rule.
• _parse_auth_pair(raw, source) — the USER:PASS split is factored out so
  both routes reject the same malformed input with the same wording, with the
  error naming which route it came from.
• --auth help text now states that it is visible to other accounts via ps
  on a shared host, so the trade-off shows up at the point of use.

The hard refusal at web/server.py:101 is untouched — a non-loopback bind still
requires auth, with no bypass. This only adds a safe way to satisfy it.

Verification

$ engram web serve --help
  --auth USER:PASS  Enable HTTP Basic auth. Required for a non-loopback --host.
                    Visible to other accounts via ps on a shared host — prefer
                    --auth-file there.
  --auth-file PATH  Read USER:PASS from the first non-empty line of PATH. Use
                    this instead of --auth on any multi-user machine.

Exercised end to end against a real store: a 600-mode credential file, a
non-loopback bind, and a browser session over the LAN.

Not in this change

• --auth-prompt (option 2 in web serve: --auth is the only credential route, and it leaks the password via the process list #10) — interactive entry, no argv/env/disk at all.
• Environment-variable route (option 3) — weaker than the file, and worth a
  separate discussion about whether it earns its place.
• Unit tests. The three paths worth covering are: a well-formed file, the
  permission warning, and the --auth / --auth-file conflict. Happy to add them
  in this PR if preferred over a follow-up.