docs: add BPM enrichment design spec

docs/superpowers/specs/2026-05-25-bpm-enrichment-design.md

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+# BPM Enrichment Design
+
+**Date:** 2026-05-25
+**Status:** Approved
+
+## Goal
+
+Add a `bpm` field to every track object returned by the MCP server, sourced from Spotify's `/audio-features` endpoint.
+
+## Scope
+
+Three tools return track objects and must be updated:
+- `get_playlist_tracks`
+- `list_saved_tracks`
+- `search_tracks`
+
+No new MCP tools. No changes to `server.py`.
+
+## Architecture
+
+### New helper: `_enrich_with_bpm(sp, tracks)`
+
+Location: `spotify_client.py`
+
+- Takes a `spotipy.Spotify` client and a list of already-parsed track dicts.
+- Extracts track IDs from each dict's `"id"` field.
+- Slices into chunks of 100 (API limit) and calls `sp.audio_features(ids)` once per chunk.
+- Matches results back by position and sets `track["bpm"]` to the rounded tempo float.
+- If the API returns `None` for a track (features unavailable), sets `bpm` to `null`.
+- Mutates the list in place; returns nothing.
+
+### Changes to track parsers
+
+Each of the three functions adds `"id": track["id"]` to the dict it builds (currently absent from the output). After the full track list is assembled, each function calls `_enrich_with_bpm(sp, results)` before returning.
+
+## Data Shape
+
+Before:
+```json
+{
+  "name": "oh baby",
+  "artists": ["LCD Soundsystem"],
+  "album": "american dream",
+  "duration_ms": 349452,
+  "duration": "5:49",
+  "popularity": null,
+  "uri": "spotify:track:53PkA8aXiwH4ppa0V0iO7o",
+  "added_at": "2026-05-22T17:41:18Z"
+}
+```
+
+After:
+```json
+{
+  "name": "oh baby",
+  "artists": ["LCD Soundsystem"],
+  "album": "american dream",
+  "duration_ms": 349452,
+  "duration": "5:49",
+  "popularity": null,
+  "uri": "spotify:track:53PkA8aXiwH4ppa0V0iO7o",
+  "id": "53PkA8aXiwH4ppa0V0iO7o",
+  "added_at": "2026-05-22T17:41:18Z",
+  "bpm": 128.0
+}
+```
+
+## Error Handling
+
+- `sp.audio_features()` may return `None` entries for tracks with no features (rare, e.g. local files, some podcasts). These get `bpm: null`.
+- Any exception from `sp.audio_features()` propagates naturally — same behaviour as the rest of the client.
+
+## API Cost
+
+- `get_playlist_tracks` / `list_saved_tracks`: 1 extra API call per 100 tracks (batched).
+- `search_tracks`: at most 1 extra call (max 50 results per search).