Ship an example settings policy, and name the variables in .env.example

#231 added the policy but nothing to copy. The repo already answers this the
same way four times over -- Caddyfile.example, deploy.example.sh,
nginx.example.conf, .env.example -- and the new feature was the one thing
configurable here with no example beside it.

settings-policy.example.json carries all three sections with the reasoning in
it, including the part worth being deliberate about: a `changes` entry
overrides a decision a reader has already made, and if you want it to stay put
regardless that is `enforced` instead. JSON has no comments, so the commentary
is in `_`-prefixed keys, which is safe because the server reads three names and
ignores everything else.

A test asserts the shipped example stays valid against the rules the parser
enforces -- unique versions, settings objects, no comment key colliding with a
real section. An example that has drifted is worse than none: somebody copies
it, the server refuses to start, and the first experience of the feature is a
crash loop.

.env.example gains the four variables, commented out, with the file form and
the inline form and the note that the file wins over the variables.

Confirmed against the real image on the deploy host rather than reasoned
about: an immutable container -- --read-only, IMMUTABLE=1, SESSION_FILE=
empty -- starts and serves the policy both with a read-only file mount and
with the environment variables alone. The feature costs nothing in
immutability, because the only thing it writes is the applied-changes stamp,
and that goes in the reader's own settings file on Stalwart like every other
setting.

settings-policy.example.json

@@ -0,0 +1,56 @@
+{
+  "_comment": [
+    "A settings policy: what this installation decides, rather than each reader.",
+    "Point at it with SETTINGS_POLICY_FILE=/etc/ihasmail/policy.json and mount it",
+    "read-only. Read once at startup, so editing it means restarting.",
+    "Delete the sections you do not want -- all three are optional, and an",
+    "installation that sets none of them behaves exactly as ihasmail always has.",
+    "Keys and values are the ones a settings export uses: configure one account",
+    "by hand, Settings > General > Export, and copy out what you care about.",
+    "Docs: https://docs.ihasmail.org/configure/#settings-your-installation-decides"
+  ],
+
+  "_defaults_comment": [
+    "A starting point for accounts that have never had settings of their own.",
+    "The reader can change any of these afterwards. An account that already",
+    "exists never sees them -- use `changes` below to reach those."
+  ],
+  "defaults": {
+    "externalSenderBanner": true,
+    "conversationMode": true
+  },
+
+  "_enforced_comment": [
+    "Reapplied on every load, and the reader cannot change them at all. Their",
+    "controls stay visible in Settings and go dead with a line saying why.",
+    "Reset, an imported settings file, and a settings file synced from a device",
+    "that predates this policy all cannot get around them."
+  ],
+  "enforced": {
+    "externalRecipientConfirm": true
+  },
+
+  "_changes_comment": [
+    "Applied once each, to everybody, including accounts that already exist --",
+    "and the reader may change them back afterwards, which sticks.",
+    "",
+    "Each entry needs a `version` that is unique in this file. It is opaque: a",
+    "timestamp sorts and never repeats, but any unique string works. Every",
+    "account remembers the versions it has had, so a change runs exactly once",
+    "per person -- not once per browser.",
+    "",
+    "Note that a change DOES override a decision a reader has already made. That",
+    "is the point of it: it reaches people who are already here. If you want it",
+    "to stay on regardless of what they do next, that is `enforced`, not this."
+  ],
+  "changes": [
+    {
+      "version": "20260902084513",
+      "settings": { "externalSenderBanner": true }
+    },
+    {
+      "version": "20261014091500",
+      "settings": { "externalLinkWarning": true }
+    }
+  ]
+}