ML-191: avoid VACUUM / scrobble refresh write collision

Set busy_timeout: 5_000 on MusicLibrary.Repo so SQLite waits up to
5 seconds on lock contention instead of throwing SQLITE_BUSY
immediately (default is 0).

Move RepoVacuum from 0 3 to 3 3 (3:03 AM London) to avoid the
deterministic collision with RefreshScrobbles which fires on the
every-5-min cadence at the hour mark.

backlog/tasks/ml-191 - Fix-SQLite-database-busy-error-in-ListeningStats.update-1-Error-4075.md

@@ -0,0 +1,61 @@
+---
+id: ML-191
+title: 'Fix SQLite "database busy" error in ListeningStats.update/1 (Error #4075)'
+status: Done
+assignee: []
+created_date: "2026-05-19 15:34"
+updated_date: "2026-05-19 15:44"
+labels:
+  - bug
+  - database
+  - production-error
+dependencies: []
+references:
+  - "https://www.sqlite.org/pragma.html#pragma_busy_timeout"
+  - "https://hexdocs.pm/ecto_sqlite3/Ecto.Adapters.SQLite3.html"
+modified_files:
+  - config/runtime.exs
+  - config/prod.exs
+priority: medium
+ordinal: 33000
+---
+
+## Description
+
+<!-- SECTION:DESCRIPTION:BEGIN -->
+
+Production error #4075: `Elixir.Exqlite.Error` "Database busy" when `ListeningStats.update/1` performs a 100-row batch `INSERT` into `scrobbled_tracks` during the daily `RefreshScrobbles` cron job at 02:00 UTC.
+
+**Root cause:** Two cron jobs collide on `MusicLibrary.Repo`:
+
+- `RepoVacuum` fires at `0 3 * * *` Europe/London (2 AM UTC during BST) and calls `MusicLibrary.Repo.vacuum()` — **acquires an exclusive lock on the entire DB file**
+- `RefreshScrobbles` fires at `*/5 * * * *` every 5 minutes (including 2 AM UTC) and calls `ListeningStats.update/1` → `Repo.insert_all` on the same Repo
+
+When VACUUM holds the exclusive lock and `insert_all` tries to write, SQLite throws `SQLITE_BUSY` immediately because **no `busy_timeout` is configured** in production `config/runtime.exs` (only test has it). The default busy_timeout is 0 = fail immediately instead of waiting.
+
+The collision is deterministic: it happens every day at 2 AM UTC (3 AM London BST). During GMT (winter), the same collision would happen at 3 AM UTC. `RefreshScrobbles` runs every 5 minutes so it always collides with any scheduled maintenance job on the same Repo.
+
+<!-- SECTION:DESCRIPTION:END -->
+
+## Acceptance Criteria
+
+<!-- AC:BEGIN -->
+
+- [x] #1 `busy_timeout: 5_000` is set for `MusicLibrary.Repo` in `config/runtime.exs` production config
+- [x] #2 `RepoVacuum` cron schedule in `config/prod.exs` is set to `3 3 * * *` (3 minutes past the hour) to avoid collision with `RefreshScrobbles` every-5-min cadence
+- [ ] #3 Error #4075 no longer occurs in production after deploy
+<!-- AC:END -->
+
+## Final Summary
+
+<!-- SECTION:FINAL_SUMMARY:BEGIN -->
+
+**Fix:** SQLite "database busy" error (#4075) on `scrobbled_tracks` INSERT during daily `RefreshScrobbles` cron job.
+
+**Root cause:** `RepoVacuum` (3 AM London = 2 AM UTC BST) runs `VACUUM` which acquires an exclusive DB lock. `RefreshScrobbles` fires every 5 minutes (`*/5`) including at the hour mark. Without `busy_timeout`, SQLite throws `SQLITE_BUSY` immediately instead of waiting.
+
+**Changes:**
+
+- `config/runtime.exs`: Added `busy_timeout: 5_000` to `MusicLibrary.Repo` — SQLite now waits up to 5s on lock contention instead of failing immediately
+- `config/prod.exs`: Moved `RepoVacuum` from `0 3 * * *` to `3 3 * * *` — VACUUM runs at :03 past the hour, 2 minutes before the next `RefreshScrobbles` at :05, avoiding the collision
+<!-- SECTION:FINAL_SUMMARY:END -->

config/prod.exs

@@ -49,8 +49,9 @@ config :music_library, Oban,
        {"0 */12 * * *", MusicLibrary.Worker.PruneAssetCache},
        # every day at 2 am,
        {"0 2 * * *", MusicLibrary.Worker.PruneAssets},
-       # every day at 3 am,
+       # every day at 3:03 am (staggered 3 min past the hour to avoid
-       {"0 3 * * *", MusicLibrary.Worker.RepoVacuum},
+       # collision with RefreshScrobbles every-5-min cadence, fixes #4075)
+       {"3 3 * * *", MusicLibrary.Worker.RepoVacuum},
        # every day at 4 am,
        {"0 4 * * *", MusicLibrary.Worker.RepoOptimize},
        # every first day of the month, staggered hourly

config/runtime.exs

@@ -105,6 +105,7 @@ if config_env() == :prod do
     # 128MB * pool_size = base memory usage
     cache_size: -128_000,
     pool_size: String.to_integer(System.get_env("POOL_SIZE") || "5"),
+    busy_timeout: 5_000,
     show_sensitive_data_on_connection_error: false
 
   background_database_path =