fix: preserve packet_feedback and created_at on packet re-save

[clates]

Summary

• Bug A (data loss): INSERT OR REPLACE in _insert_weekly_packet triggers a SQLite-internal DELETE + INSERT on primary key conflict. With FOREIGN KEY ... ON DELETE CASCADE on packet_feedback, this silently deletes all user-submitted feedback on every packet re-save. Since generate_trio_for_student is called as a background task after every feedback submission, feedback was being erased immediately after submission.
• Bug B (audit timestamp): Both created_at and updated_at were set to now on every call, overwriting the original creation timestamp on re-saves.

Changes

• src/packet_store.py — Replace INSERT OR REPLACE INTO weekly_packets with INSERT INTO ... ON CONFLICT(packet_id) DO UPDATE SET .... The upsert updates the row in-place without deleting it, so the ON DELETE CASCADE to packet_feedback never fires. created_at is intentionally excluded from the DO UPDATE SET list so it is only written on the first insert.
• src/packet_store.py — Add conn.execute("DELETE FROM daily_lessons WHERE packet_id = ?", ...) in save_weekly_packet before _persist_daily_lessons. This explicit delete replaces the implicit cascade cleanup that the old DELETE approach provided, since _persist_daily_lessons uses plain INSERT (not INSERT OR IGNORE) and would fail on UNIQUE constraint for (packet_id, day_label) without it.
• tests/test_packet_store.py — Add test_resave_preserves_packet_feedback: saves a packet, saves feedback, re-saves the packet, asserts feedback is still present. Add test_resave_does_not_overwrite_created_at: saves a packet, re-saves it, asserts created_at is unchanged.

Test plan

• test_resave_preserves_packet_feedback — confirms feedback rows survive a packet re-save
• test_resave_does_not_overwrite_created_at — confirms created_at is immutable after first insert
• Existing test_save_weekly_packet_persists_rows — confirms initial save still writes all rows correctly
• Existing test_save_weekly_packet_rolls_back_on_error — confirms transaction atomicity is preserved
• CI runs on Python 3.12 where datetime.UTC is available (local env is 3.10; import error is pre-existing)

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