feat: ability to use postgres as store for starter distro (#4076)

## What does this PR do?

The starter distribution now comes with all the required packages to
support persistent stores—like the agent store, metadata, and
inference—using PostgreSQL. Users can enable PostgreSQL support by
setting the `ENABLE_POSTGRES_STORE=1` environment variable.

This PR consolidates the functionality from the removed `postgres-demo`
distribution into the starter distribution, reducing maintenance
overhead.

**Closes: #2619**  
**Supersedes: #2851** (rebased and updated)

## Changes Made

1. **Added PostgreSQL support to starter distribution**
   - New `run-with-postgres-store.yaml` configuration
- Automatic config switching via `ENABLE_POSTGRES_STORE` environment
variable
   - Removed separate `postgres-demo` distribution

2. **Updated to new build system**
   - Integrated postgres switching logic into Containerfile entrypoint
   - Uses new `storage_backends` and `storage_stores` API
   - Properly configured both PostgreSQL KV store and SQL store

3. **Updated dependencies**
   - Added `psycopg2-binary` and `asyncpg` to starter distribution
   - All postgres-related dependencies automatically included

## How to Use

### With Docker (PostgreSQL):
```bash
docker run \
  -e ENABLE_POSTGRES_STORE=1 \
  -e POSTGRES_HOST=your_postgres_host \
  -e POSTGRES_PORT=5432 \
  -e POSTGRES_DB=llamastack \
  -e POSTGRES_USER=llamastack \
  -e POSTGRES_PASSWORD=llamastack \
  -e OPENAI_API_KEY=your_key \
  llamastack/distribution-starter
```

### PostgreSQL environment variables:
- `POSTGRES_HOST`: Postgres host (default: `localhost`)
- `POSTGRES_PORT`: Postgres port (default: `5432`)
- `POSTGRES_DB`: Postgres database name (default: `llamastack`)
- `POSTGRES_USER`: Postgres username (default: `llamastack`)
- `POSTGRES_PASSWORD`: Postgres password (default: `llamastack`)

## Test Plan

All pre-commit hooks pass (mypy, ruff, distro-codegen)  
`llama stack list-deps starter` confirms psycopg2-binary is included  
Storage configuration correctly uses PostgreSQL backends  
Container builds successfully with postgres support  

## Credits

Original work by @leseb in #2851. Rebased and updated by @r-bit-rry to
work with latest main.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Sébastien Han @leseb

---------

Signed-off-by: Sébastien Han <seb@redhat.com>
Co-authored-by: Sébastien Han <seb@redhat.com>

docs/docs/distributions/self_hosted_distro/starter.md

@@ -163,7 +163,41 @@ docker run \
   --port $LLAMA_STACK_PORT
 ```
 
-### Via venv
+The container will run the distribution with a SQLite store by default. This store is used for the following components:
+
+- Metadata store: store metadata about the models, providers, etc.
+- Inference store: collect of responses from the inference provider
+- Agents store: store agent configurations (sessions, turns, etc.)
+- Agents Responses store: store responses from the agents
+
+However, you can use PostgreSQL instead by running the `starter::run-with-postgres-store.yaml` configuration:
+
+```bash
+docker run \
+  -it \
+  --pull always \
+  -p $LLAMA_STACK_PORT:$LLAMA_STACK_PORT \
+  -e OPENAI_API_KEY=your_openai_key \
+  -e FIREWORKS_API_KEY=your_fireworks_key \
+  -e TOGETHER_API_KEY=your_together_key \
+  -e POSTGRES_HOST=your_postgres_host \
+  -e POSTGRES_PORT=your_postgres_port \
+  -e POSTGRES_DB=your_postgres_db \
+  -e POSTGRES_USER=your_postgres_user \
+  -e POSTGRES_PASSWORD=your_postgres_password \
+  llamastack/distribution-starter \
+  starter::run-with-postgres-store.yaml
+```
+
+Postgres environment variables:
+
+- `POSTGRES_HOST`: Postgres host (default: `localhost`)
+- `POSTGRES_PORT`: Postgres port (default: `5432`)
+- `POSTGRES_DB`: Postgres database name (default: `llamastack`)
+- `POSTGRES_USER`: Postgres username (default: `llamastack`)
+- `POSTGRES_PASSWORD`: Postgres password (default: `llamastack`)
+
+### Via Conda or venv
 
 Ensure you have configured the starter distribution using the environment variables explained above.
 
@@ -171,8 +205,11 @@ Ensure you have configured the starter distribution using the environment variab
 # Install dependencies for the starter distribution
 uv run --with llama-stack llama stack list-deps starter | xargs -L1 uv pip install
 
-# Run the server
+# Run the server (with SQLite - default)
 uv run --with llama-stack llama stack run starter
+
+# Or run with PostgreSQL
+uv run --with llama-stack llama stack run starter::run-with-postgres-store.yaml
 ```
 
 ## Example Usage