Add speaker setup, Spotify, and architecture documentation

[timvw]

Summary

Adds three documentation files covering real-world deployment knowledge from a SoundTouch 20 (firmware 27.0.6).

Closes #172, #173, #174

Files

docs/speaker-setup.md (closes #172)

• SSH access on firmware 27.x (USB stick method — the old TAP command is removed)
• macOS junk file gotchas (.fseventsd, .Spotlight-V100)
• Making SSH persistent (/mnt/nv/remote_services)
• Extracting speaker data (port 8090 API vs SSH)
• Redirecting speaker to soundcork (editing SoundTouchSdkPrivateCfg.xml)
• Warnings: TAP console dangers, read-only filesystem

docs/spotify.md (closes #173, relates to #159)

• Two Spotify systems explained (Spotify Connect vs SoundTouch integration)
• Spotify preset fix: kick-start via Spotify Connect
• Why it works (ZeroConf authentication, session in RAM)
• Traffic analysis evidence
• Preset management via local API (port 8090)

docs/architecture.md (closes #174)

• Bose server status as of February 2026 (BMX/updates already 404ing, marge alive)
• Shutdown date: May 6, 2026 (extended from Feb 18)
• Links to official Bose EOL page and published API docs
• Local vs proxy mode explanation
• Circuit breaker design
• Data flow diagrams (power-on, TuneIn playback, Spotify)

[colinmacgiolla]

Just worth noting as I used it today, Wi-Fi vs. Wired is not a factor.

It's very fussy about the USB stick. 8GB / FAT32 / remote_services file only failed, 1GB / FAT32 / remote_services file only succeeded.