initial: demo spotify OAuth

API.md

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+## API
+
+- reverse proxy (protects API keys)
+	- download song (spotmate.online API)
+		- /songs
+	- fetch library (spotify.com API)
+		- /
+	- fetch playlist (spotify.com API)
+	- nice-to: fetch playlist icon, song icon
+
+	- register user: (username, service)
+	- upload diagnostics
+.
+
+- do we abstract API from Spotify?
+	- allows other services
+	-
+	- needs a user concept {user}.{service}
+.
+
+- do we cache the mp3 of the songs?
+	- will we later use server-side processing of songs?
+	- ideas about 50 ms centered gaussian-window, and 2048-point STFT
+	- get features and train discriminator to find beats in song
+	- actual beats where user stepped
+	-
+	- possibly we will need a time environment (spanning a musical bar?) as feature
+	- cf. songs like "Disturbed - Rise" with strong complex drum pattern
+.
+
+- first draft:
+	- constant (0-th order) BPM estimate of song - step pattern derived
+	- adapt tempo only (not phase synchronized)
+.
+