feat: send periodic flood adverts at a configurable interval

[sg]

Summary

Adds a periodic flood-advert feature: the bot sends a flood advertisement every N hours (0 = disabled). The interval is seeded from mcbot.conf on first run, then stored in the database (bot_meta) and managed at runtime — changes persist across restarts and take effect within ~60s without a restart. Settable via !adm advert interval <hours> and the web Manage→Radio screen.

Behavior

• !adm advert interval 3 → flood advert every 3 hours; !adm advert interval 0 disables; bare !adm advert interval shows the current value.
• The first periodic advert is one full interval out (setting it won't trigger an immediate flood while tuning).
• Any flood advert (manual !adm advert flood, web button, or periodic) resets the clock, so the guarantee is "≥ N hours between flood adverts."

Changes

• mcbot.py — Config.advert_interval_hours ([bot], default 0) + parse; MCBot state, _load_advert_interval() (DB-authoritative, seeds from config when the key is absent), set_advert_interval() (memory + DB, resets schedule), send_advert() anchors the schedule on flood, and a periodic_advert task (60s poll, registered for shutdown).
• management.py — radio_advert_interval() (read) + radio_set_advert_interval() (validate 0–168h, persist, audit radio.advert_interval).
• commands/adm.py — !adm advert interval <hours> subcommand + help text.
• webapi/routers/dbadmin.py — GET/POST /api/radio/advert-interval.
• web-ui/src/views/Manage.vue — interval input + Apply in the Radio Advert section, auto-loaded on tab open; static assets rebuilt.
• mcbot.conf.example — documents advert_interval_hours as the first-run seed.
• tests/test_advert_interval.py — seed-from-config, DB-authoritative-over-config, set-persists/reset, mgmt validation + audit, flood-advert anchoring.

Testing

tests/test_advert_interval.py passes; existing suites (logging, repeat-tracking, eviction, decode-advert) still pass; py_compile clean; npm run build clean.