feat: all-in-one embedded single-container mode (#377)

Embedded Postgres 17 + Redis via s6-overlay when DATABASE_URL/REDIS_URL are unset; restores the one-command docker run for 2.0. EMBEDDED=0 disables; Compose stays the production path. Verified arm64 (14/14 lifecycle + Compose regression) and amd64 (build + embedded smoke).

docker/s6/s6-rc.d/redis/run

@@ -0,0 +1,8 @@
+#!/command/with-contenv sh
+exec s6-setuidgid snapotter redis-server \
+  --dir /data/redis \
+  --bind 127.0.0.1 \
+  --port 6379 \
+  --maxmemory-policy noeviction \
+  --appendonly yes \
+  --maxmemory "${REDIS_MAXMEMORY:-512mb}"