feat(db): add excluded_problems store to permanently filter not-relevant problems

[smithrashell]

Problem

When a user skips a problem with reason not_relevant, the current implementation only weakens the graph relationships (-0.8). This is probabilistic — the problem can still resurface in future sessions because weakening edges reduces selection likelihood but does not prevent reselection.

There is no deterministic way to permanently exclude a problem today.

Why Not Existing Stores

• standard_problems — pure LeetCode reference data (id, title, slug, difficulty, tags). Must not hold user data.
• problems — user attempt-backed records. A problems record is only created when an attempt is created, so a record is not guaranteed to exist at skip time. A user can see a problem in their session and skip it as not_relevant before ever submitting an attempt.
• problem_relationships — graph edges between two problems. A single problem can have N relationship records (one per connection). Writing a flag here requires N writes and N reads to answer a simple yes/no question.
• user_actions — append-only event log (autoIncrement key). Reconstructing current state from events is the wrong data shape for a flag lookup.

Proposed Solution

Add a dedicated excluded_problems store.

Schema

keyPath: leetcode_id        ← O(1) lookup at session assembly time
fields:
  leetcode_id  number       ← primary key, matches standard_problems.id
  excluded_at  string       ← ISO timestamp
  reason       string       ← 'not_relevant' (extensible if other exclusion flows added later)

No additional indexes needed beyond the keyPath — lookups will always be by leetcode_id.

DB Version Bump

Current version: 47 → new version: 48

A migration entry is required in src/shared/db/migrations/ and storeCreation.js.

Changes Required

1. DB layer

• Add createExcludedProblemsStore() to src/shared/db/core/storeCreation.js
• Register the store in the STORES array and validateDatabaseIntegrity required stores list in src/shared/db/index.js
• Add migration for version 48 in src/shared/db/migrations/
• Add read/write helpers (e.g. excludeProblem(leetcodeId, reason), isExcluded(leetcodeId), getExcludedIds()) to a new src/shared/db/stores/excludedProblems.js

2. Skip handler

In src/background/handlers/problemHandlers.js — applySkipReasonEffects:

case 'not_relevant':
  if (hasRelationships) {
    const weakenResult = await weakenRelationshipsForNotRelevant(leetcodeId);
    result.graphUpdated = weakenResult.updated > 0;
  }
  await excludeProblem(leetcodeId, 'not_relevant');   // ← add this
  await SessionService.skipProblem(leetcodeId);
  break;

Graph weakening is kept as a secondary signal. The store write is the hard guarantee.

3. Session assembly

Filter excluded problems before building the session. The exact location depends on where standard_problems records are fetched and filtered for session selection — excludeProblem IDs should be loaded once and applied as a filter at that point.

Outcome

Scenario	Before	After
Problem skipped as not_relevant	Graph weakened, may resurface	Graph weakened + permanently excluded
Problem reappears after not_relevant skip	Possible	Impossible
Lookup cost at session assembly	N/A	O(1) per problem via keyPath