feat(selfhost): warn loudly at boot when SQLite has no acknowledged backup

[JSONbored]

Summary

A single SQLite file with no backup is a data-loss SPOF — yet /ready still answers 200, so an operator can run with zero durability believing they're healthy.

At boot the container now logs a loud selfhost_backup_advisory warning when on the SQLite backend without BACKUP_ACKNOWLEDGED=true, pointing to the Litestream sidecar setup. Wire a backup, then set BACKUP_ACKNOWLEDGED=true to silence it. Postgres backends skip the warning (back up Postgres instead). The advisory is a pure, tested helper (sqliteBackupAdvisory) in health.ts; the bootstrap wiring is in the codecov-ignored server.ts; the docs §6 backup section is hardened to call this out.

Validation

• npm run typecheck; npm run test:coverage — full suite green; helper 100% covered (SQLite+no-ack → warn; acknowledged → null; Postgres → null).

Roadmap item #8 (data safety). Note: actively verifying Litestream replication from the container would couple to Litestream's metrics endpoint; the loud boot advisory + docs is the honest, dependency-free guardrail.

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.44%. Comparing base (95fe9a4) to head (cda8ee7).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1488   +/-   ##
=======================================
  Coverage   95.44%   95.44%           
=======================================
  Files         194      194           
  Lines       21078    21080    +2     
  Branches     7625     7626    +1     
=======================================
+ Hits        20118    20120    +2     
  Misses        383      383           
  Partials      577      577           

Files with missing lines	Coverage Δ
src/selfhost/health.ts	94.11% <100.00%> (+0.78%)	⬆️

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