Rebased version of #968 (which was itself a rebase of #905) — resolves
merge conflict with #906 (clock-skew UI) that landed on master.
## Conflict resolution
**`public/observers.js`** — master (#906) added "Clock Offset" column to
observer table; #968 split "Last Seen" into "Last Status" + "Last
Packet" columns. Combined both: the table now has Status | Name | Region
| Last Status | Last Packet | Packets | Packets/Hour | Clock Offset |
Uptime.
## What this PR adds (unchanged from #968/#905)
- `last_packet_at` column in observers DB table
- Separate "Last Status Update" and "Last Packet Observation" display in
observers list and detail page
- Server-side migration to add the column automatically
- Backfill heuristic for existing data
- Tests for ingestor and server
## Verification
- All Go tests pass (`cmd/server`, `cmd/ingestor`)
- Frontend tests pass (`test-packets.js`, `test-hash-color.js`)
- Built server, hit `/api/observers` — `last_packet_at` field present in
JSON
- Observer table header has all 9 columns including both Last Packet and
Clock Offset
## Prior PRs
- #905 — original (conflicts with master)
- #968 — first rebase (conflicts after #906 landed)
- This PR — second rebase, resolves#906 conflict
Supersedes #968. Closes#905.
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