docs(ops): stop the compose header contradicting itself, and name the four variables step 2 dropped (#204)

Three corrections a review of #196 found, all in text #196 itself rewrote.

**The compose header asserted something it falsified three lines later.** "All must be set in Portainer for this stack. All are secrets except DEMO_MODE" — but three PayPal entries are unused while `DEMO_MODE` is `true`, three more are marked Optional, and `SMTP_FROM` is not a secret. The runbook sends the operator to that exact block as authoritative, so an operator cutting over during the demo interim reads "all must be set", has no live PayPal credentials — which is the whole reason the interim exists — and either stops or invents a `BACKUP_PASSPHRASE`, which is how you get archives nobody can decrypt. The blanket claim is gone; each entry already says whether it is required and when.

**Step 2 enumerated nine of the thirteen interpolated names.** `USPS_CLIENT_ID`, `USPS_CLIENT_SECRET`, `UPLOADS_BASE_URL` and `BACKUP_PASSPHRASE` were missing. The USPS pair is the one that matters, and it now gets a sentence of its own: losing it is the only silent failure in this step. Address validation is skipped when those are empty rather than failing, so checkout keeps working and quietly stops validating addresses, with no crash loop and nothing in step 7 that would notice. The list also now says to take everything the stack holds rather than working from the list, because an enumeration reads as a checklist however it is introduced.

**The `DB_PASSWORD` failure was described wrongly, and its error names a variable the operator never typed.** It does not fail to authenticate against its data directory — the app never reaches a connection attempt, `checkAlwaysRequired` refuses at boot, and the message says `PGPASSWORD` because the compose file injects it as `PGPASSWORD=${DB_PASSWORD}`. An operator grepping for `DB_PASSWORD` finds nothing. That is now stated.

The crash-loop examples were reordered to match the case the section claims to be about. It headlines "the likeliest outcome of a missed step 2", but the only block shown was the PayPal triple, which cannot occur during the demo interim, and #196 replaced it with a `DEMO_MODE`-only block that is not what a missed step 2 produces either. A wholesale miss is two problems led by `PGPASSWORD`; that is now first, with the `DEMO_MODE`-only and mistyped-value forms after it.

Every quoted line was verified by running the real validator against production's entry set rather than by reading the source — `2 problem(s)` and `1 problem(s)` counts included — and all four now match character for character. The mistyped-value message is quoted in full rather than truncated, which was the point of the complaint that produced it.

`DEMO_MODE` is no longer called "the only one that is not a secret", which was false of `SMTP_FROM` and `UPLOADS_BASE_URL`. It is the only one that is a setting rather than a credential, which is true and a better hook.

Verified: 278 backend unit tests pass, including the compose guard that parses this file.

Closes #204
Refs #196

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docker-compose.prod.yml

@@ -57,8 +57,10 @@
 # (#190). Everything else is written out, so there is one place to look and one
 # thing that can be wrong.
 #
-# Required stack environment variables. All must be set in Portainer for this
-# stack. All are secrets except DEMO_MODE:
+# The stack environment variables this file reads. Each entry says whether it is
+# required and when — there is no blanket rule, because three are unused while
+# DEMO_MODE is `true`, three are optional, and DEMO_MODE and SMTP_FROM are not
+# secrets at all:
 #
 #   DEMO_MODE             `true` or `false`, exactly. Whether real payments are
 #                         taken. Not a secret — it is here rather than written