log: support d (day) unit in roll_interval log rotation directive

[mohammed90]

Originally fixed in #7865

roll_interval was using time.ParseDuration which doesn't understand the d (day) unit, causing roll_interval 1d to fail at startup despite Caddy's duration conventions supporting it.

Changes

• FileWriter.RollInterval: Changed from time.Duration to caddy.Duration — also fixes JSON config support for the d unit
• UnmarshalCaddyfile: Use caddy.ParseDuration instead of time.ParseDuration for roll_interval parsing
• OpenWriter: Cast caddy.Duration → time.Duration when passing to timberjack's RotationInterval
• Tests: Added TestCaddyfile_RollInterval covering 1d, 24h, 30m, 1.5d, and invalid input

log {
    output file /var/log/caddy/access.log {
        roll_interval 1d   # previously failed; now works
    }
}

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