Reclassify nudge-shown (MARS-600) from warn to info severity to stop real-time alert paging

nudge-shown (MARS-600 profile-photo feature, 3 legs: reviewer /practicas/[id], student /mi-cursada/[id], app-wide ProfilePhotoNag) is currently logged at appEvents.severity=warn, which routes it through scrp-applog-mars's real-time/frequent alert channel. All three legs have been confirmed benign repeatedly over multiple days (reference_mars600_three_legs) -- it is expected friction telemetry, never an error. Per the fleet PLACEMENT PRINCIPLE (project_error_alerting_suppress_standard: app-side classifier writes the row at a NON-alerting severity at write time, SSOT is appEvents.severity), reclassify nudge-shown's write-time severity from warn to info so it stops paging PM in real time. Do NOT stop logging it -- MARS-69 friction epic wants the volume data, just off the frequent-alert tier. Same shape as the already-adopted suppress list (login-unknown, bot-scanner 404, etc.) -- this class was simply never added. Elazar flagged the real-time paging as a direct cost (every no-op DM burns a CC turn) and wants it stopped, not just triaged faster.