fix(activity): stop unmapped actions rendering as identical events

[ssavutu]

Uploading a file and deleting it produced the same row in the activity log — "Activity event" over the filename, distinguishable only by timestamp. Approving and deleting a comment had the same problem.

Cause

Not the logging. The server records media_uploaded and media_deleted as distinct actions and always has — confirmed against the activity table. ACTION_META in the activity view simply had no media_* or comment_* keys, so both fell through to a FALLBACK_META constant whose label, icon and colour were fixed. Every unmapped action collapsed into one indistinguishable row.

Changes

• Six missing mappings: media_uploaded, media_updated, media_deleted, media_index_started, comment_status_updated, comment_deleted.
• The constant fallback becomes a label derived from the action key, so an action nobody remembers to map reads as "Media uploaded" rather than losing its meaning. This is the actual fix — the bug wasn't a missing entry, it was that a missing entry destroyed information.
• The breakdown panel had the same blind spot inverted: it enumerated ACTION_META and counted matching events, so unmapped actions were absent from it entirely. Now counts from the events themselves, busiest first.

Verification

Cross-checked the mapping against every action string the handlers emit, including those built into variables in taxonomy.go and handlers.go: nothing emitted is unmapped, nothing mapped is dead.

Follow-up

classified_moderated / classified_deleted are deliberately unmapped here — those handlers don't exist on main yet. Once the classifieds PR merges they render via the new fallback as "Classified moderated": correct, but with a generic icon. Worth a small follow-up adding explicit entries.

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