feat(etl): Wave 4C — /api/etl/status route + CLI command

[0bserver07]

Summary

Wave 4C ships the ETL pipeline health surface — one endpoint + one CLI
command — so the dashboard can show a status badge and CLI users get a
one-line health check.

• GET /api/etl/status — watcher state, mart watermarks vs the max
  usage_events.id, per-provider event counts, per-cost-source
  breakdown, and a health enum (live / syncing / stale / error).
  <50ms response on a 200K-event store; all counts are indexed
  COUNT(*).
• stackunderflow etl status [--format text|json] — same payload,
  rendered as text by default. Works without a running server.
• Shared assembler at stackunderflow/etl/status.py so both surfaces
  call the same SQL. The CLI degrades gracefully when no live watcher
  handle is available (reports running=\"unknown\").

Health rules (from the wave-4C spec)

• error — a mart is more than 100 events behind and the watcher reports running=false.
• stale — a mart is more than 100 events behind, watcher still alive (catch-up may still happen).
• syncing — lag > 0 and a refresh ran in the last 10 seconds.
• live — zero lag, or no events at all.

Test plan

• 17 new tests pass (10 route, 7 CLI) covering response shape, every
  health transition, and watcher graceful-degrade.
• pytest tests/ -q clean: 1489 passed, 2 skipped (was 1472 before).
• HTTP smoke test against the user's real store: 200 OK, 5.9ms wall
  time end-to-end.
• CLI smoke test: `stackunderflow etl status` correctly reports
  `stale` (watcher state unknown, marts 150K events behind) on the
  user's machine where ingest is current but the marts haven't been
  refreshed.

Files touched

• new: `stackunderflow/routes/etl.py` (thin FastAPI shell)
• new: `stackunderflow/etl/status.py` (shared assembler — keeps SQL out of the CLI)
• new: `tests/stackunderflow/routes/test_etl_status.py`
• new: `tests/stackunderflow/cli/test_etl_status.py`
• modified: `stackunderflow/server.py` (register router — 2 lines)
• modified: `stackunderflow/cli.py` (add `etl status` subcommand)
• modified: `docs/cli-reference.md` (new `## ETL Commands` section)
• modified: `docs/api-reference.md` (new `### GET /api/etl/status`)
• modified: `CHANGELOG.md`