feat: always serve UI dist and fail fast when missing

[lan17]

Summary

This PR makes FastAPI always serve the built UI and fail fast at startup when UI artifacts are missing.

Why

We want a single predictable runtime mode:

• Python server always serves UI
• no runtime toggle drift
• immediate startup failure if UI build artifacts are not present

What changed

• Removed server.serve_ui from config model and env parsing.
• Kept server.ui_dist_dir with env override via HOMESEC_SERVER_UI_DIST_DIR.
• Enforced strict config keys for FastAPIServerConfig (extra = "forbid") so legacy serve_ui now fails validation.
• Updated UI serving behavior in FastAPI:
  • Always configures SPA routes.
  • Raises startup error if index.html is missing.
  • Raises startup error if assets/ directory is missing.
  • Adds non-GET catch-all (POST/PUT/PATCH/DELETE/OPTIONS) to return 404 for unknown SPA paths.
• Docker:
  • Removed HOMESEC_SERVER_SERVE_UI env from Dockerfile.
  • Kept HOMESEC_SERVER_UI_DIST_DIR=/app/ui/dist default.
• Tests:
  • Updated API/config tests for fail-fast semantics and removed toggle assumptions.
  • Added tests/homesec/ui_dist_stub.py helper to keep unrelated route tests deterministic when using default ui/dist in stubs.
• Regenerated UI API artifacts:
  • ui/src/api/generated/openapi.json
  • ui/src/api/generated/schema.ts

Breaking change

server.serve_ui is removed. Any config still including it will now fail validation.

Validation

• make check
• docker build --target runtime -t homesec:ui-dist-check .

Related

• Notion ticket #77: https://www.notion.so/314d8336c59f818aa7a8effab9ac46e0

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.73%. Comparing base (daf65c3) to head (7ecaa24).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #38      +/-   ##
==========================================
- Coverage   82.73%   82.73%   -0.01%     
==========================================
  Files         107      107              
  Lines        9685     9673      -12     
==========================================
- Hits         8013     8003      -10     
+ Misses       1672     1670       -2     

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