Add OPTIM.md — comprehensive optimization plan

[theovilardo]

Motivation

• Add a single, repository-level document that diagnoses performance hotspots and provides a prioritized, exhaustive plan to improve CPU, RAM, IO and UI fluency based on current code observations (player UI, sync worker, DB queries, metadata and album art handling, dual ExoPlayer usage).

Description

• Add OPTIM.md containing a deep diagnosis, prioritized optimization actions (batching sync, FTS5 search, caching strategies, UI recomposition reductions, metadata and album-art improvements, low-end audio mode, etc.), estimated impact ranges for each action, and references to the source files used for the analysis.

Testing

• No automated tests were run because this is a documentation-only change; the change is limited to creating OPTIM.md and does not modify runtime code.

---

Codex Task