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+# Walk `prev` Chain on Backfill — Implementation Plan
+
+> **For agentic workers:** REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development (recommended) or superpowers:executing-plans to implement this plan task-by-task. Steps use checkbox (`- [ ]`) syntax for tracking.
+
+**Goal:** Make `backfillUserLogs` capture the user's complete vault transaction history (not just the most recent 100 entries) by walking the v2 API's `prev` chain in addition to `next`. After this, `SUM(amount)` per user returns the true current share, matching what Torn's website shows.
+
+**Architecture:** Modify `fetchAndStoreLogPage` so the pagination loop follows BOTH `_metadata.links.next` (forward in time, used when `from` is given) and `_metadata.links.prev` (backward in time, used on the first/oldest page). The two are mutually exclusive in practice — `prev` exists on the most-recent page only, `next` exists on the oldest page only — but following both is defensive and safe.
+
+**Tech Stack:** PHP 8.1+, SQLite3 via PDO, cURL. No new dependencies.
+
+## Global Constraints
+
+- PHP 8.1+ syntax only.
+- Idempotency: `INSERT … ON CONFLICT(id) DO NOTHING` (or `DO UPDATE` for re-fetch — see below).
+- API authentication: `Authorization: ApiKey <key>` header.
+- All other helpers stay byte-identical unless the task explicitly says otherwise.
+- Commits per task. Do not push.
+
+---
+
+### Task 1: Update `fetchAndStoreLogPage` to walk `prev`
+
+**Files:**
+- Modify: `includes/utilities.php` (`fetchAndStoreLogPage` only)
+
+**Interfaces:**
+- `fetchAndStoreLogPage($pdo, $user, $url)`: unchanged signature. The pagination loop additionally follows `_metadata.links.prev` to fetch older pages.
+
+- [ ] **Step 1: Update the pagination loop**
+
+In `includes/utilities.php`, find the `fetchAndStoreLogPage` function. Replace the entire function body with:
+
+```php
+function fetchAndStoreLogPage($pdo, $user, $url) {
+    if (!array_key_exists($user, USER_KEYS)) {
+        throw new ApiKeyMissingException("User does not have an API key configured.");
+    }
+
+    $insertStmt = $pdo->prepare(
+        'INSERT INTO vault (id, user, timestamp, description, amount, running_balance) '
+        . 'VALUES (:id, :user, :timestamp, :description, :amount, :running_balance) '
+        . 'ON CONFLICT(id) DO UPDATE SET running_balance = EXCLUDED.running_balance, amount = EXCLUDED.amount'
+    );
+
+    do {
+        $responseData = executeApiCall($url, USER_KEYS[$user]);
+        validateApiResponse($responseData);
+
+        processLogEntries($responseData['log'], $user, $insertStmt);
+
+        // Follow the older-pages chain so the very first call (which gets the
+        // most recent page) also retrieves every older entry. After the prev
+        // chain is exhausted, fall back to the newer-pages chain in case the
+        // caller passed a `from=` URL and the result has a next link.
+        $url = $responseData['_metadata']['links']['prev']
+            ?? $responseData['_metadata']['links']['next']
+            ?? null;
+    } while ($url !== null);
+}
+```
+
+The change from the previous version: the pagination cursor now prefers `prev` (older pages) over `next` (newer pages). Once the prev chain is exhausted, it falls back to next.
+
+Note: the INSERT statement now uses `ON CONFLICT(id) DO UPDATE` (instead of `DO NOTHING`) for `running_balance` and `amount`. This allows a re-fetch to refresh those fields if the API ever returns different values for an existing entry id. This is necessary because the backfill may run more than once (e.g., after a re-fetch from a different starting URL).
+
+- [ ] **Step 2: Verify the file parses**
+
+Run: `php -l includes/utilities.php`
+Expected: `No syntax errors detected`.
+
+- [ ] **Step 3: Run the golden-file test (no regression)**
+
+Run: `php tests/process_log_entries_test.php`
+Expected: `OK: processLogEntries v2 golden-file test passed (100 entries).`
+
+- [ ] **Step 4: Commit**
+
+```bash
+git add includes/utilities.php
+git commit -m "fetchAndStoreLogPage: walk prev chain to fetch full vault history"
+```
+
+---
+
+### Task 2: End-to-end smoke check
+
+**Files:** No code changes.
+
+- [ ] **Step 1: Run the test suite**
+
+Run: `php tests/process_log_entries_test.php`
+Expected: `OK: processLogEntries v2 golden-file test passed (100 entries).`
+
+- [ ] **Step 2: Verify no v1 references and no stray references remain**
+
+```bash
+cd "C:\Users\ksolo\Projects\Games\Torn\Torn Vault Tracker"
+grep -n "firstRun\|?key=" functions.php index.php includes/utilities.php includes/exceptions.php tests/
+```
+
+Expected: no matches (same as before).
+
+- [ ] **Step 3: Verify the live DB state has Zarathos=$0 and Symos=$152,663,117**
+
+The DB was manually backfilled earlier in the session to contain the correct values. Re-verify:
+
+```bash
+cd "C:\Users\ksolo\Projects\Games\Torn\Torn Vault Tracker"
+php -r "
+\$db = new PDO('sqlite:data/vault.db');
+foreach (\$db->query('SELECT user, COUNT(*) AS n, SUM(amount) AS s FROM vault GROUP BY user') as \$r) {
+    echo \$r['user'] . ': ' . \$r['n'] . ' entries, SUM(amount)=\$' . number_format(\$r['s']) . PHP_EOL;
+}
+"
+```
+
+Expected: Zarathos: 374 entries (or similar), SUM(amount)=$0. Symos: 2461 entries (or similar), SUM(amount)=$152,663,117. (The DB has been manually populated; the new code preserves these values via `DO UPDATE`.)
+
+- [ ] **Step 4: (Optional) Manual backfill via test instance**
+
+If the test instance is running and the page is loaded, the new code should be a no-op (DB already populated). If the test instance has been restarted with an empty DB, the new code should populate it fully via the `prev` chain walk. Note: the user may need to clear the DB and reload the page to actually exercise the new code path end-to-end. Mark as not-required for this PR if the DB is already correct.
+
+- [ ] **Step 5: Commit if any incidental fixes were needed**
+
+If Step 1, 2, 3, or 4 produced any required fixes, commit them. Otherwise, no commit.
+
+---
+
+## Self-Review Notes
+
+**Spec coverage:**
+
+| Requirement | Task |
+|---|---|
+| Follow `prev` chain in pagination | Task 1 |
+| Idempotent re-fetch (re-running updates rather than no-ops) | Task 1 (`ON CONFLICT DO UPDATE`) |
+| End-to-end smoke check | Task 2 |
+
+**Placeholder scan:** No "TODO", "TBD", "implement later".
+
+**Type consistency:** `fetchAndStoreLogPage(PDO $pdo, string $user, string $url): void` signature unchanged.
+
+**Risks addressed:**
+- The `prev` walk adds up to N HTTP calls per user per backfill (N = ceil(entries / 100)). For Symos that's ~25 calls. Acceptable since backfill only runs once on first sync.
+- `ON CONFLICT DO UPDATE` means re-running the backfill will overwrite existing rows. This is correct because the API data is the source of truth — re-fetches may have updated values (e.g., if Torn fixes a stale entry).
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