docs: README in the standard order; setup files say what works today (#6277)

README: logo header, short summary, a compose quick start with one-line
comments, first-run steps, a configuration table, setup pitfalls,
updating and backups, then features grouped by what you do. The Specs
section (it called the Android client the "Flutter companion app"), the
dated tag-history sentence and the Fable pointer are gone. Feature lines
were checked against the code: radio seeds from a track only, and the
Android app plays from its cache rather than offering downloads.

.env.example used the SMARTMUSIC_ prefix, which the config loader has not
read for a long time, so copying it set nothing. It now uses MINSTREL_.

config.example.yaml is also the config the image bakes in at
/etc/smartmusic/config.yaml. It listed five recommendation weights
(base_weight, like_boost, recency_weight, skip_penalty, jitter_magnitude)
that moved to Admin > Tuning; yaml.Unmarshal ignored them silently. They
are removed, and the file parses to exactly Default(), so the running
server is unchanged. coverart_from_mbcaa is not listed: nothing reads it.

docker-compose.yml, the Makefile and the spec banners lose their task and
milestone ids.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

config.example.yaml

@@ -1,82 +1,54 @@
-# Minstrel example configuration.
-# Every field is optional; defaults shown below match Default() in internal/config.
-# Each setting can be overridden at runtime via MINSTREL_* environment variables
-# (see internal/config/config.go for the full list).
+# Minstrel example configuration. In the container image, mount it at
+# /etc/smartmusic/config.yaml. Every field is optional; the values below are
+# the defaults. Each MINSTREL_* variable named here overrides its field.
 
 server:
-  address: ":4533"
+  address: ":4533"            # MINSTREL_SERVER_ADDRESS
 
 storage:
-  # On-disk root for cached runtime artifacts (currently playlist cover
-  # collages under <data_dir>/playlist_covers/). Default: "./data" relative
-  # to the working directory. Production deployments should set an absolute
-  # path on persistent storage.
-  # Env: MINSTREL_STORAGE_DATA_DIR
-  data_dir: "./data"
+  # Artwork, caches and the stream-signing key. Use persistent storage.
+  data_dir: "./data"          # MINSTREL_STORAGE_DATA_DIR (the image sets /app/data)
 
 database:
-  # Postgres connection string, e.g. postgres://minstrel:minstrel@localhost:5432/minstrel?sslmode=disable
-  url: ""
+  # e.g. postgres://minstrel:minstrel@localhost:5432/minstrel?sslmode=disable
+  url: ""                     # MINSTREL_DATABASE_URL
 
 log:
-  level: "info"    # debug | info | warn | error
-  format: "text"   # text | json
+  level: "info"               # debug | info | warn | error. MINSTREL_LOG_LEVEL
+  format: "text"              # text | json. MINSTREL_LOG_FORMAT
 
 library:
-  # Filesystem roots to scan for music. Each path is walked recursively.
-  # These are the starting folders only: once they are changed in
-  # Admin › Library, the saved ones are used and this value is ignored.
-  # Changing a folder there moves its tracks with it, so moving the library
-  # to a new mount keeps every play, like and playlist.
-  # Env: MINSTREL_LIBRARY_SCAN_PATHS (colon-separated, PATH-style)
-  scan_paths: []
-  # Kick off a scan on server startup. On by default so a fresh install
-  # populates its library on first boot with no manual step. Scans are
-  # incremental (unchanged files are skipped by mtime), so the cost on
-  # every later restart is just a directory walk, not a full re-index.
-  # Set false if you'd rather scan only on a schedule / via the admin UI.
-  # Env: MINSTREL_LIBRARY_SCAN_ON_STARTUP
-  scan_on_startup: true
+  # Library folders, walked recursively. Used on first start only: once they
+  # are changed in Admin › Library, the saved ones win. Changing a folder there
+  # moves its tracks with it, so plays, likes and playlists survive a new mount.
+  scan_paths: []              # MINSTREL_LIBRARY_SCAN_PATHS (colon-separated)
+  # Scan on startup. Scans are incremental, so a restart only walks the folders.
+  scan_on_startup: true       # MINSTREL_LIBRARY_SCAN_ON_STARTUP
+  # Contact sent to the Cover Art Archive in the User-Agent. Empty sends
+  # the project's URL.
+  contact_email: ""           # MINSTREL_CONTACT_EMAIL
 
-# Per-instance branding. Operators can rename their instance ("Family
-# Jukebox", "Office Music", etc.) and override the OG share-preview
-# description. Both fields default to sensible Minstrel values when
-# unset; you only need this section to override.
-# Env: MINSTREL_BRANDING_APP_NAME, MINSTREL_BRANDING_DESCRIPTION
 branding:
-  app_name: "Minstrel"
-  description: "Self-hosted music server with Subsonic compatibility, smart shuffle, and music acquisition."
+  app_name: "Minstrel"        # MINSTREL_BRANDING_APP_NAME
+  description: "Self-hosted music server with Subsonic compatibility, smart shuffle, and music acquisition."  # MINSTREL_BRANDING_DESCRIPTION
 
 subsonic:
-  # Gate for the legacy `p=` plaintext password parameter on /rest/*.
-  # Leave disabled unless a specific client requires it — Minstrel prefers
-  # apiKey (OpenSubsonic) and falls back to u+t+s token auth.
-  # Env: MINSTREL_SUBSONIC_ALLOW_PLAINTEXT_PASSWORD
-  allow_plaintext_password: false
+  # Accept the plaintext `p=` password on /rest/*. Leave off unless a client
+  # needs it; apiKey and token auth work without it.
+  allow_plaintext_password: false   # MINSTREL_SUBSONIC_ALLOW_PLAINTEXT_PASSWORD
 
 events:
-  # Idle gap (minutes) between play_events that closes a play_session and
-  # starts a new one. Spec §6.
+  # Minutes of silence that end a listening session.
   session_timeout_minutes: 30
-  # Skip rule (spec §6): a play counts as a SKIP if completion ratio is below
-  # this threshold AND duration played (ms) is below the next threshold.
+  # A play is a skip when it stops before this fraction of the track AND
+  # before this many milliseconds.
   skip_max_completion_ratio: 0.5
   skip_max_duration_played_ms: 30000
 
 recommendation:
-  # Base score every candidate gets before adjustments. Spec §6.
-  base_weight: 1.0
-  # Bonus for tracks the user has liked (general_likes).
-  like_boost: 2.0
-  # Multiplier on recency_decay (1.0 for tracks ≥ 30 days stale, 0 for fresh).
-  recency_weight: 1.0
-  # Penalty multiplier on skip_ratio (skips/plays); 1.0 = full penalty.
-  skip_penalty: 1.0
-  # ± random jitter applied to every candidate; breaks ties without dominating.
-  jitter_magnitude: 0.1
-  # Hard-suppression window: tracks played within this many hours never appear.
+  # Scoring weights are tuned in Admin › Tuning, not here.
+  # Tracks played within this many hours are left out of radio.
   recently_played_hours: 1
-  # Default radio size when ?limit= is not specified.
+  # Radio length when ?limit= is not given, and the most ?limit= may ask for.
   radio_size: 50
-  # Maximum allowed ?limit= (server caps to this regardless).
   radio_size_max: 200