Add peo4c: neo4j-browser on PostgreSQL + Apache AGE

A single container image with:
- neo4j-browser built from source and served on :7474
- PostgreSQL with Apache AGE (based on apache/age)
- a Python bridge speaking Bolt 4.4/5.0-5.4 over TCP and WebSocket on
  :7687 that runs Cypher through AGE's cypher() function, emulates the
  Neo4j procedures neo4j-browser relies on, and maps databases to AGE
  graphs

Includes workarounds for AGE limitations (parameters are inlined as
literals, `id(x) IN [...]` and the browser's expand-node query are
rewritten) and an end-to-end smoke test using the Neo4j Python driver.

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

docker/initdb/10-peo4c.sh

@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+# Prepares a freshly initialised cluster for peo4c.
+set -euo pipefail
+
+# The bridge connects over 127.0.0.1 with the credentials users log in with, so
+# loopback TCP connections must check passwords. The postgres image trusts them
+# by default; Unix-socket connections (psql inside the container) stay trusted.
+sed -i -E 's/^(host[[:space:]]+[^[:space:]]+[[:space:]]+[^[:space:]]+[[:space:]]+(127\.0\.0\.1\/32|::1\/128)[[:space:]]+)trust/\1scram-sha-256/' \
+    "$PGDATA/pg_hba.conf"
+
+# Create the default AGE graph that neo4j-browser opens on first connect.
+psql -v ON_ERROR_STOP=1 --username "$POSTGRES_USER" --dbname "${POSTGRES_DB:-$POSTGRES_USER}" \
+    -v graph="${PEO4C_DEFAULT_GRAPH:-neo4j}" <<'EOSQL'
+LOAD 'age';
+SET search_path = ag_catalog, "$user", public;
+SELECT ag_catalog.create_graph(:'graph')
+WHERE NOT EXISTS (SELECT 1 FROM ag_catalog.ag_graph WHERE name = :'graph');
+EOSQL

docker/peo4c-entrypoint.sh

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+#!/usr/bin/env bash
+# Runs PostgreSQL (through the stock postgres entrypoint) and the peo4c bridge
+# side by side, and stops the container when either of them exits.
+set -euo pipefail
+
+if [ "${1:-}" != "postgres" ]; then
+    exec docker-entrypoint.sh "$@"
+fi
+
+export PEO4C_PG_DATABASE="${PEO4C_PG_DATABASE:-${POSTGRES_DB:-${POSTGRES_USER:-postgres}}}"
+
+docker-entrypoint.sh "$@" &
+pg_pid=$!
+
+cd /opt/peo4c/bridge
+gosu postgres python3 -m peo4c &
+bridge_pid=$!
+
+shutdown() {
+    kill -TERM "$bridge_pid" 2>/dev/null || true
+    kill -TERM "$pg_pid" 2>/dev/null || true
+}
+trap shutdown TERM INT
+
+# Wait until one of the two processes exits, then stop the other one too.
+set +e
+wait -n "$pg_pid" "$bridge_pid"
+status=$?
+shutdown
+wait "$pg_pid" 2>/dev/null
+exit "$status"