fix: keep a resolve no louder than its incident for ntfy, Gotify and Pushover - #592

Fixes #591

A resolve could be louder than the incident it closed: ntfy always sent resolves at 3, Gotify omitted the priority so the server default applied, and Pushover sent resolves at 0 even when the configured priority was negative.

• Shared rule in transport.ResolvePriority(configured, override, normal): an explicit override wins, otherwise the configured priority is capped at the provider's normal level; an unset priority behaves as before.
• New optional alert.<provider>.resolvedPriority for ntfy (1-5), Gotify (0-10) and Pushover (-2 to 1).
• Normal levels: ntfy 3, Gotify 5, Pushover 0. Unset Gotify priority stays omitted.
• Pushover: the emergency receipt cancel is unchanged. resolvedPriority of 2 (or out of range) is rejected at startup, like an invalid priority. For ntfy and Gotify an out-of-range value is logged and ignored.
• Shared transport.IntSetting replaces the duplicated float64/int/int64 switch in ntfy, Gotify, Pushover and ClickUp.
• Docs and provider catalog said the ntfy default priority is 4; the code leaves it unset (ntfy uses 3). Docs and catalog corrected, code default unchanged.

Behaviour change: setups with a low priority now get equally quiet resolves, and Gotify setups with a priority above 5 get resolves capped at 5 instead of the server default.