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/postgres-init/type

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+oneshot

docker/s6/s6-rc.d/postgres-init/up

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+#!/command/with-contenv sh
+/usr/local/bin/embedded-postgres-bootstrap.sh

docker/s6/s6-rc.d/postgres-ready/type

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+oneshot

docker/s6/s6-rc.d/postgres-ready/up

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+/command/with-contenv sh -c "until pg_isready -h 127.0.0.1 -p 5432 -q; do sleep 1; done"

docker/s6/s6-rc.d/postgres/down-signal

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+SIGINT

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

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+#!/command/with-contenv sh
+exec s6-setuidgid postgres /usr/lib/postgresql/17/bin/postgres -D /data/postgres

docker/s6/s6-rc.d/postgres/type

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+longrun

docker/s6/s6-rc.d/redis-ready/type

@@ -0,0 +1 @@
+oneshot

docker/s6/s6-rc.d/redis-ready/up

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+/command/with-contenv sh -c "until redis-cli -h 127.0.0.1 -p 6379 ping 2>/dev/null | grep -q PONG; do sleep 1; done"

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

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+#!/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}"

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

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+longrun

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

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+#!/command/with-contenv sh
+cd /app
+exec s6-setuidgid snapotter pnpm --filter @snapotter/api run start

docker/s6/s6-rc.d/snapotter/type

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+longrun