Read spending from zenmoney in the poller, answer it from facts (#125)

The trust boundary is zenmoney, not maven — they already hold his bank
sessions. So the poller reads /v8/diff/ and writes totals as
facts(kind=env, source=poll:zenmoney); core reads those back when he asks
and never sees the token.

internal/zenmoney sums transactions per currency over a window, skipping
tombstoned rows and transfers between his own accounts, and refuses to
encode a summary built from zero transactions. That refusal is the whole
design: a failed or empty read writes nothing and leaves the last good
total alone, because a zero recited as fact is worse than silence. No
currency conversion either — a figure he can check against his bank beats
one he cannot.

Off unless configured, and the token is read from a FILE rather than a
flag so it never lands in `ps`, in docker-compose.yml, or in shell
history. Nothing about the money is search input, no tick rule reads the
keys, and the log lines name keys, never figures.

The live-credential half is BLOCKED: there is no zenmoney account or token
here, so everything is verified against a recorded diff fixture.

docker-compose.yml

@@ -101,9 +101,17 @@ services:
               "-netdata", "http://127.0.0.1:19999",
               "-kuma", "http://127.0.0.1:3001/metrics",
               "-kuma-key", "uk5_mavpoll-key"]
+              # Money tracking (Vikunja #125) is OFF: it needs a zenmoney token,
+              # which mavpoll reads from a FILE so it never appears in `ps`, in
+              # this file, or in shell history. To enable, mount the token and
+              # append: "-zenmoney-token-file", "/run/secrets/zenmoney.token"
+              # (optionally "-zenmoney-interval", "1h"). Core never sees the
+              # token — the poller writes facts(kind=env, source=poll:zenmoney)
+              # and mavend only reads those back when he asks.
     depends_on: [mavend]
     volumes:
       - sockets:/run/maven
+      # - ./deploy/zenmoney.token:/run/secrets/zenmoney.token:ro
 
 volumes:
   dbdata: