ML-151: document Assets.Cache TTL and invalidation strategy

Expand @moduledoc with cache key structure, 7-day TTL via
@one_week_seconds, periodic pruning by PruneAssetCache (every 12h),
and rationale for TTL-only invalidation (content-addressable immutable
assets). Update architecture.md entry accordingly.

backlog/tasks/ml-151 - Document-Assets.Cache-TTL-and-cache-invalidation-strategy.md

@@ -1,9 +1,10 @@
 ---
 id: ML-151
 title: Document Assets.Cache TTL and cache invalidation strategy
-status: To Do
+status: Done
 assignee: []
 created_date: '2026-04-30 10:48'
+updated_date: '2026-04-30 12:04'
 labels:
   - documentation
   - assets
@@ -28,7 +29,35 @@ Add a `@moduledoc` to `Assets.Cache` that explains:
 
 ## Acceptance Criteria
 <!-- AC:BEGIN -->
-- [ ] #1 `Assets.Cache` `@moduledoc` documents the TTL value and where it is configured
-- [ ] #2 The invalidation strategy (TTL-based expiry) is explained with rationale
-- [ ] #3 Any architectural implications are captured in `docs/architecture.md`
+- [x] #1 `Assets.Cache` `@moduledoc` documents the TTL value and where it is configured
+- [x] #2 The invalidation strategy (TTL-based expiry) is explained with rationale
+- [x] #3 Any architectural implications are captured in `docs/architecture.md`
 <!-- AC:END -->
+
+## Implementation Plan
+
+<!-- SECTION:PLAN:BEGIN -->
+## Plan
+
+### What we're documenting
+
+`Assets.Cache` is an ETS-based cache for transformed binary asset data. Assets are stored by SHA256 hash and are immutable (write-once). The cache key is `{payload, format}`.
+
+### Key facts to document
+
+- **TTL**: `@one_week_seconds` = 7 days, defined as a module attribute
+- **Pruning**: `prune/0` defaults to 7-day TTL, called every 12h by `PruneAssetCache` Oban cron worker
+- **Invalidation strategy**: TTL-based expiry only — no explicit invalidation
+
+### Rationale
+
+Explicit invalidation isn't needed because:
+1. Assets are content-addressable (SHA256) and immutable — updates create new hashes, so old entries are never requested again
+2. Periodic pruning (every 12h) cleans up stale entries
+3. ETS table is in-memory, cleared on restart
+
+### Changes
+
+1. Expand `@moduledoc` in `lib/music_library/assets/cache.ex`
+2. Expand the `Assets.Cache` entry in `docs/architecture.md` Business Logic Modules table
+<!-- SECTION:PLAN:END -->