fix: resolve 8 Claude-identified bugs (null guards, UTC, fp precision, cascade deletes)

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Summary

Fixes all 8 bugs identified by automated codebase analysis (issues #77–#84). Also updates AGENTS.md to enforce PR-only workflow going forward.

• [Bug] NULL progress_blob crashes get_filtered_standards in logic.py #77 / [Bug] NULL progress_blob causes 500 on /progress-map endpoint #78 / [Bug] NULL plan_rules_blob crashes generate_weekly_plan for new students #79 — NULL progress_blob / plan_rules_blob caused TypeError crashes for new students in logic.py, main.py, and agent.py. Fixed with or "{}" null guards.
• [Bug] generate_weekly_plan uses local time instead of UTC, causing wrong week_of on non-UTC servers #80 — generate_weekly_plan used datetime.now() (local time) instead of UTC, producing wrong week_of dates on non-UTC servers. Fixed to datetime.now(UTC).
• [Bug] grade_level defaults to 0 when student has no packets, breaking initial plan generation #81 — _get_grade_level fell back to 0 when a student had no packets, causing plan generation to fail with "no standards found". Now falls back to the grade level in the student's metadata blob.
• [Bug] Floating-point drift in reverse_quantity_feedback accumulates over multiple feedback cycles #82 — Repeated apply/reverse quantity feedback cycles accumulated floating-point drift in activity_bias. Fixed by rounding to 6 decimal places after each update.
• [Bug] Concurrent plan generation creates duplicate weekly packets (delete-then-insert race) #83 — Concurrent plan saves for the same student could create duplicate packet rows. Replaced DELETE + INSERT with atomic INSERT OR REPLACE.
• [Deficiency] No CASCADE DELETE on packet_feedback — student deletion leaves orphaned feedback rows #84 — Deleting a student left orphaned packet_feedback rows. delete_student() now deletes the student's weekly_packets first, which cascades to all packet-related data.

Also adds section 7 to AGENTS.md documenting the correct automated analysis → issue → PR workflow to prevent future direct-to-main commits.

Test plan

• Create a new student (NULL blobs) and verify plan generation and /progress-map no longer return 500
• Verify student deletion removes associated packets and feedback from the DB
• Submit feedback multiple times and confirm activity_bias doesn't drift
• Confirm week_of in generated plans is always a Monday in UTC

Closes #77, #78, #79, #80, #81, #82, #83, #84

🤖 Generated with Claude Code

[clates]

Review — Critical regression + 4 additional findings

This PR fixes legitimate bugs but introduces one regression and has several issues that need to be addressed before merge.

[CRITICAL] Dual packet_feedback schema — ingest_standards.py wins, packet_store.py's version never applies

ingest_standards.py creates packet_feedback with:

FOREIGN KEY (student_id) REFERENCES student_profiles(student_id)
-- no ON DELETE CASCADE, no FK to weekly_packets

packet_store.py's ensure_schema() creates packet_feedback with:

FOREIGN KEY(packet_id) REFERENCES weekly_packets(packet_id) ON DELETE CASCADE
-- no FK to student_profiles

Both use CREATE TABLE IF NOT EXISTS. Because ingest_standards.py is the DB initialization script and runs first, its schema wins — and ensure_schema() in packet_store.py is a no-op for packet_feedback. This has two consequences that this PR's changes make worse:

1. delete_student now raises IntegrityError (see inline comment on line 255 in db_utils.py)
2. The cascade packet_store.py relies on for child-row cleanup never fires in production

The fix here should be to either (a) remove the duplicate schema from ingest_standards.py and add a migration, or (b) make both files agree on the FK constraints.

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[CRITICAL — regression] PRAGMA foreign_keys = ON activates the FK constraint from ingest_standards.py's packet_feedback schema:

FOREIGN KEY (student_id) REFERENCES student_profiles(student_id)

This has no ON DELETE CASCADE and the weekly_packets delete above does not cascade to packet_feedback (because in the actual live DB, packet_feedback has an FK to student_profiles, not to weekly_packets). So the sequence is:

1. DELETE FROM weekly_packets WHERE student_id = ? — orphans packet_feedback rows (no cascade)
2. DELETE FROM student_profiles WHERE student_id = ? — raises sqlite3.IntegrityError: FOREIGN KEY constraint failed because packet_feedback rows still reference this student_id

Any student who has ever submitted feedback is now undeletable after this PR. The endpoint returns 500 instead of 204.

Suggestion: rather than enabling PRAGMA here, explicitly delete packet_feedback rows before deleting student_profiles (similar to how weekly_packets is already handled), or remove the conflicting FK from ingest_standards.py and rely solely on packet_store.py's schema.

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[MEDIUM] This except sqlite3.OperationalError: pass is too broad. It was intended to handle "no such table: weekly_packets" in minimal test DBs, but it will also silently swallow:

• A disk-full error
• A locked database
• A corrupted DB page
• A misspelled table name after a future rename

If the goal is to handle missing tables in tests, consider either:

except sqlite3.OperationalError as e:
    if "no such table" not in str(e):
        raise

or better, fix the test fixtures to always call ensure_schema() before delete_student.

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[HIGH] INSERT OR REPLACE on a primary-key conflict internally does DELETE + INSERT. With PRAGMA foreign_keys = ON (set in _get_connection()), the cascade from weekly_packets fires — meaning all packet_feedback rows for this packet are silently deleted every time a packet is re-saved for the same week.

Note: this was also true with the old _delete_existing_packet DELETE approach, so this is not a regression from this PR specifically — but the PR's changes are a good time to fix it.

If a student submits feedback and then the weekly automation runs again (which trio_generator triggers on every feedback submission via background_tasks.add_task), the feedback is permanently lost with no error.

A proper fix would check whether packet_feedback rows already exist for this packet_id before replacing the packet, or change the approach to INSERT INTO ... ON CONFLICT(packet_id) DO UPDATE SET ... WHERE excluded.status != 'completed' to avoid clobbering completed packets entirely.

Also note: created_at is always set to now (lines 199–200 in the new file), so every re-save overwrites the original packet creation timestamp. If preservation matters, use ON CONFLICT DO UPDATE SET updated_at = excluded.updated_at instead.

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[LOW] The fallback sentinel changed from 0 to 1. While better than returning 0 blindly, grade level 0 is Kindergarten per the standards schema. A new Kindergarten student whose metadata_blob has no grade_level key, or whose profile has no metadata at all, will get grade-1 lesson plans generated instead of grade-0 ones.

The metadata_fallback path added above handles the common case correctly. This final fallback (return 1) is fine if grade 0 students are always guaranteed to have grade_level in their metadata blob — but that guarantee isn't enforced anywhere.