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>

.env.example

@@ -1,12 +1,11 @@
-# Minstrel environment variable overrides.
-# Copy to .env for docker compose, or export before `go run ./cmd/minstrel`.
-# Values shown are sensible defaults for the local docker-compose postgres.
+# Environment for running the server outside Docker (`go run ./cmd/minstrel`).
+# The values match the Postgres in docker-compose.yml.
 
-SMARTMUSIC_SERVER_ADDRESS=:4533
-SMARTMUSIC_DATABASE_URL=postgres://minstrel:minstrel@localhost:5432/minstrel?sslmode=disable
-SMARTMUSIC_LOG_LEVEL=info
-SMARTMUSIC_LOG_FORMAT=text
+MINSTREL_SERVER_ADDRESS=:4533
+MINSTREL_DATABASE_URL=postgres://minstrel:minstrel@localhost:5432/minstrel?sslmode=disable
+MINSTREL_LOG_LEVEL=info
+MINSTREL_LOG_FORMAT=text
 
-# Set this when running `go test ./...` without -short to exercise the
-# migration test against a running postgres.
-# MINSTREL_TEST_DATABASE_URL=postgres://minstrel:minstrel@localhost:5432/minstrel?sslmode=disable
+# Runs the database tests in `go test ./...` (without -short). Use a separate
+# database: the tests truncate every table.
+# MINSTREL_TEST_DATABASE_URL=postgres://minstrel:minstrel@localhost:5432/minstrel_test?sslmode=disable