v1.2.6 — friendlier first run

Lessons from a thorough beta review of a sibling project, applied here before anyone hit them:

• Plans seed themselves on first boot — no more easy-to-miss manual seed step; --force re-seeding still available.
• /health reports readiness — plan count, readings count, newest reading, and what to do next on a fresh install, instead of a bare {"ok": true} over an empty dashboard.
• README simplified — upload your HDF on the Settings page; the API path remains for automation. Python 3.12+ noted for non-Docker use.
• Exact dependency pins — python-multipart==0.0.* was effectively unpinned and it handles the upload path.