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>

docker-compose.yml

@@ -1,17 +1,8 @@
 version: "3.9"
 
-# Local development environment for Minstrel.
-#
-# Integration tests run against a SEPARATE database (minstrel_test) so a
-# test run never truncates the dev `minstrel` DB (Fable #339). Use the
-# Makefile target, which ensures the test DB exists then points the
-# tests at it:
-#   make test-integration
-# (CI runs the same suite against its own ephemeral Postgres service —
-# see the `integration` job in .gitea/workflows/release.yml.)
-#
-# Full stack (server + db):
-#   docker compose up --build
+# Local development stack: `docker compose up --build`.
+# The database tests use a separate minstrel_test database, so they never
+# truncate your dev data. Run them with `make test-integration`.
 
 services:
   postgres:
@@ -26,8 +17,8 @@ services:
     #   - "5432:5432"
     volumes:
       - minstrel-pgdata:/var/lib/postgresql/data
-      # Creates minstrel_test on a fresh volume (Fable #339). No-op on
-      # an existing volume — make test-integration ensures it instead.
+      # Creates minstrel_test on a fresh volume. On an existing volume,
+      # make test-integration creates it instead.
       - ./deploy/initdb:/docker-entrypoint-initdb.d:ro
     healthcheck:
       test: ["CMD-SHELL", "pg_isready -U minstrel -d minstrel"]
@@ -51,12 +42,9 @@ services:
     ports:
       - "4533:4533"
     volumes:
-      # Point ./music at your test library (symlink, bind-mount, whatever).
-      # Read-only so a buggy scanner can't rewrite your files.
+      # Your test library, read-only so a scanner bug can't change your files.
       - /mnt/Media/Music:/music:ro
-      # Cached artifacts: playlist cover collages, artist art, album-cover
-      # fallbacks (when the music dir is RO). Persists across container
-      # recreates so the operator doesn't redownload art on every up/down.
+      # Artwork and caches, kept across container recreates.
       - minstrel-data:/app/data
 
 volumes: