fix: save the observation sent to POST /entry

[PPeitsch]

Description

POST /entry accepted an observation field but never stored it, although ScheduleEntry has the column (review finding C10). Now:

• the observation is saved, trimmed, and an empty string clears it;
• a request without the key keeps the note already saved for the day;
• GET /entry/<date> returns observation.

The create/update branches are merged into one path, and the unused imports in the module are gone.

Type of Change

• Bug fix

Testing

• New test_manual_entry_saves_observation (save, keep when omitted, clear with empty string).
• pytest (255 passed), black, isort, mypy clean.

Screenshots

Not applicable: JSON endpoint, no UI change.

Checklist

• I have read the contributing guidelines
• I have followed the project's code style (Black, isort, PEP 8)
• My code generates no new warnings

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ All tests successful. No failed tests found.

Files with missing lines	Coverage Δ
app/routes/manual_entry.py	98.27% <100.00%> (ø)

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