fix(config): preflight /config writability and surface it clearly (#278)

[stevezau]

Summary

Fixes #278. A read-only /config (host permission mismatch — common on TrueNAS where the dataset isn't owned by the container's PUID/PGID) surfaced as SQLite's opaque attempt to write a readonly database in two confusing ways:

• Scans looked frozen — the scan ran, but every job-state persist to jobs.db failed, flooding the log once per item while the UI never updated.
• Schedule creation failed writing scheduler.db, and the catch-all blamed "a malformed cron expression or a clash with an existing schedule" — the wrong cause entirely.

Changes

• web/config_health.py (new) — probe_config_health(): authoritative create+delete probe, plus /proc/mounts (tells ownership vs :ro mount apart), statvfs (low-space), and network-FS (NFS/SMB) detection — each with an actionable fix. Probe file is uniquely named per attempt so concurrent gthread probes (shared PID) can't race into a false "unwritable".
• web/app.py — startup preflight logs one clear error at boot instead of a per-item flood.
• routes/api_system.py — GET /api/system/config-health (cheap, no GPU) feeding the banner; folds in live media-mount health.
• routes/api_jobs.py — 503 guard on create_job/create_manual_job: refuses to start a scan that can't persist, with the fix.
• routes/api_schedules.py — detects the readonly-DB case → 503 + chown hint instead of the misleading cron 500.
• templates/base.html + static/js/app.js — global banner on every page: red when /config isn't writable, yellow for network-FS / low-space / missing media mount.

Verification

• New tests/test_config_health.py — 12 tests (probe matrix incl. :ro mount, low-space, network-fs, cleanup-race regression; the two schedule error cells; both 503 guards).
• Full suite: 3227 passed, 83.6% coverage.
• Live UI check (throwaway instance, real read-only /config): the red banner renders with the exact chown hint; POST /api/jobs and POST /api/schedules both return 503 with the fix; banner clears on the next poll once perms are fixed (no restart). Architecture Review run pre-commit (1 MED + 2 LOW, all fixed).

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 87.23404% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.06%. Comparing base (f8a2933) to head (de8c72e).
⚠️ Report is 1 commits behind head on dev.

Files with missing lines	Patch %	Lines
media_preview_generator/web/config_health.py	87.23%	12 Missing ⚠️
media_preview_generator/web/routes/api_system.py	77.27%	5 Missing ⚠️
media_preview_generator/web/routes/api_jobs.py	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #279      +/-   ##
==========================================
+ Coverage   84.00%   84.06%   +0.06%     
==========================================
  Files          89       90       +1     
  Lines       19669    19807     +138     
==========================================
+ Hits        16523    16651     +128     
- Misses       3146     3156      +10     

Files with missing lines	Coverage Δ
media_preview_generator/web/app.py	83.38% <100.00%> (+0.10%)	⬆️
...edia_preview_generator/web/routes/api_schedules.py	89.17% <100.00%> (+3.17%)	⬆️
media_preview_generator/web/routes/api_jobs.py	86.41% <93.33%> (+0.11%)	⬆️
media_preview_generator/web/routes/api_system.py	82.85% <77.27%> (-0.31%)	⬇️
media_preview_generator/web/config_health.py	87.23% <87.23%> (ø)

... and 2 files with indirect coverage changes

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