fix(admin): a cleared poll-interval part no longer emits "null"

[ajslater]

Implements P3f of tasks/followups-implementation-plan.md §5.6 — the last item in the plan.

Stacked on #888 → #887 → #886 → #885. Retarget as each merges.

The bug

duration-input.vue's three v-number-inputs each call update(), which emits fieldsToDjangoDuration. Clearing a part sets its model to null, and:

String(null)              // "null"  -- four characters
"null".padStart(3, "0")   // "null"  -- already longer than 3, so untouched

…so the emit became "null 01:00:00". Its consumer is library-create-update-inputs.vue for Library.poll_every, a DurationField, which rejects it with a 400 — the same 400 the dialog rendered as raw JSON, because nothing parsed the envelope's error shape (#888).

Reproduced: reverting only the composition guard makes the test fail with expected 'null 01:00:00' not to contain 'null'.

Two halves, because rules alone are not enough

Per-part rules, bound to the :min/:max already on each control ([0, 365] / [0, 23] / [0, 59]). Vuetify's $required alias carves out v === 0, so zero — a legitimate value here — still passes, and the parts sit inside the dialog's <v-form ref="form"> so form.validate() disables Submit.

A composition guard, because rules do not stop the emit. A cleared, negative or unparseable part coerces to zero. Coerced rather than refusing to emit: the field still reads empty to the user, the rule still says so, and Submit stays disabled — but whatever does get emitted is a valid duration.

The rule arrays are frozen module constants read from data, not computed — they are static.

Tests

frontend/tests/unit/duration-input.test.js, 7 cases — none existed. Initial parsing, normal composition, a cleared part never emitting "null", every part cleared still composing "000 00:00:00", the coercion table, zero being a legitimate value rather than an empty one, and the bounds matching the controls.

make fix && make lint && make ty clean. Full make test green: 1283 pytest, 602 vitest (+7).

NEWS, under Fixes: "Clearing part of a poll interval no longer produces an invalid value."

Note on the other rider

§5.6 also carried the password-message wording (five sites, one wording). That shipped with #876, which already touched the one line that disagreed — §5.6 explicitly permits folding it there if that PR was still open, and it was.

This completes the plan

All thirteen items are now open as PRs: #876 #877 #878 #879 #880 #881 #882 #883 #884 #885 #886 #887 #888 and this one.

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