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>

README.md

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+# peo4c
+
+[neo4j-browser](https://github.com/neo4j/neo4j-browser) running on PostgreSQL, using
+[Apache AGE](https://github.com/apache/age) as the graph engine.
+
+Everything ships in one container image:
+
+```
+ neo4j-browser (static files)      peo4c bridge (Python)              PostgreSQL + AGE
+ ┌──────────────────────┐  Bolt   ┌──────────────────────────┐  SQL  ┌────────────────┐
+ │ in your web browser  │───────▶│ Bolt server (TCP + WS)    │──────▶│ cypher('g',$$ │
+ │ served on :7474      │ :7687  │ Cypher → AGE translation   │ :5432 │   ... $$)      │
+ └──────────────────────┘        │ emulated Neo4j procedures │       └────────────────┘
+                                 └──────────────────────────┘
+```
+
+## Quick start
+
+```sh
+podman build -t peo4c .
+podman run -d --name peo4c -e POSTGRES_PASSWORD=secret \
+    -p 7474:7474 -p 7687:7687 -p 5432:5432 \
+    -v peo4c-data:/var/lib/postgresql peo4c
+```
+
+Open <http://localhost:7474/> and log in with your **PostgreSQL** credentials:
+user `postgres`, password the value of `POSTGRES_PASSWORD`. Bolt clients (the Neo4j
+drivers, cypher-shell) can connect to `neo4j://localhost:7687` the same way, and
+`psql` works on port 5432.
+
+`docker` works as a drop-in replacement for `podman` in the commands above.
+
+## How it works
+
+- **Databases are AGE graphs.** Every AGE graph shows up as a Neo4j database, and
+  `:use <name>` switches between them. A graph named `neo4j` is created on first start
+  (`PEO4C_DEFAULT_GRAPH`). `CREATE DATABASE x` and `DROP DATABASE x` on the `system`
+  database call `create_graph` / `drop_graph`; AGE requires names of at least 3 characters.
+- **Queries** are sent to AGE as `SELECT * FROM cypher('<graph>', $$ <query> $$) AS (...)`.
+  The bridge works out the result columns from the final `RETURN` clause and inlines
+  query parameters as Cypher literals.
+- **Neo4j procedures** that neo4j-browser relies on (`dbms.components`, `SHOW DATABASES`,
+  `db.labels`, `db.schema.visualization`, `SHOW PROCEDURES`, ...) are emulated from the
+  AGE catalog.
+- **Workarounds for AGE**: neo4j-browser's "expand node" query uses a pattern
+  comprehension, and its relationship lookup uses `id(n) IN [...]`, which AGE rejects
+  for lists of more than one element. The bridge rewrites both into equivalent Cypher
+  that AGE accepts.
+- **Authentication** is passed straight through: every Bolt connection opens its own
+  PostgreSQL connection with the credentials you log in with.
+
+## Configuration
+
+| Variable | Default | Purpose |
+| --- | --- | --- |
+| `POSTGRES_PASSWORD` | (required) | Password of the PostgreSQL superuser, see the [postgres image docs](https://hub.docker.com/_/postgres). `POSTGRES_USER` and `POSTGRES_DB` work too. |
+| `PEO4C_DEFAULT_GRAPH` | `neo4j` | Graph created at initialisation and used when a client does not pick a database. |
+| `PEO4C_PG_DATABASE` | `$POSTGRES_DB` | PostgreSQL database holding the graphs. |
+| `PEO4C_ADVERTISED_BOLT_PORT` | `7687` | Bolt port advertised to the browser; set it when you publish 7687 on another host port. |
+| `PEO4C_ADVERTISED_HOST` | `localhost` | Host reported in `SHOW DATABASES` and to clients that do not send a routing address. |
+| `PEO4C_LOG_LEVEL` | `INFO` | `DEBUG` logs every generated SQL statement. |
+
+Build arguments: `AGE_IMAGE` (base image, default `apache/age:release_PG18_1.8.0`) and
+`NEO4J_BROWSER_REF` (neo4j-browser commit to build).
+
+## Limitations
+
+This is a first version. Things that do not work (yet):
+
+- `RETURN *`, `EXPLAIN` and `PROFILE`.
+- Write statistics: write queries report "(no changes, no records)" even when they
+  changed the graph.
+- Anything AGE does not support, for example multiple labels per node, pattern
+  comprehensions, temporal and spatial types, indexes and constraints through Cypher.
+- User and role management, `SHOW TRANSACTIONS`, `:sysinfo`.
+- TLS (`bolt+s://`, HTTPS).
+- Each query's full result is fetched from PostgreSQL before it is streamed to the
+  client, so very large results use memory in the bridge.
+
+## Development
+
+The bridge is a dependency-light Python package in [`bridge/peo4c`](bridge/peo4c)
+(only `psycopg` 3). To run it outside the container against any AGE-enabled
+PostgreSQL:
+
+```sh
+pip install 'psycopg[binary]'
+cd bridge
+PEO4C_PG_HOST=localhost PEO4C_PG_DATABASE=postgres PEO4C_BROWSER_DIR=/path/to/neo4j-browser/dist \
+    python -m peo4c
+```
+
+[`tests/smoke_test.py`](tests/smoke_test.py) runs end-to-end checks through the official
+Neo4j Python driver against a running instance:
+
+```sh
+pip install neo4j
+python tests/smoke_test.py neo4j://localhost:7687 postgres secret
+```