Config file loader with Zod schema and env var overrides

## Parent PRD

#1660

## What to build

Implement the core config file loading mechanism for Graph Explorer. The server reads `GE_CONFIG` env var (defaulting to `./graph-explorer.config.json`), parses the JSON, and validates it with a Zod schema. When no file is found, the server starts with empty defaults (no connections). `GE_*` environment variables (`GE_PORT`, `GE_LOG_LEVEL`, `GE_LOG_FORMAT`, `GE_TLS`, `GE_TLS_CERT`, `GE_TLS_KEY`) override their corresponding config file values.

The Zod schema should cover the full config shape defined in #1660: `server` block (port, logLevel, logFormat, tls, corsOrigin), `connections` (keyed by stable ID, with name, url, queryEngine, fetchTimeout, nodeExpansionLimit, authentication as a discriminated union), and `feedbackUrl`. All fields optional with sensible defaults.

This replaces the current `env.ts` validation, `dotenv` loading, and `CONFIGURATION_FOLDER_PATH` logic.

## Acceptance criteria

- [ ] Reads config from `GE_CONFIG` env var path, falls back to `./graph-explorer.config.json`
- [ ] Parses and validates JSON with Zod schema matching the shape in #1660
- [ ] Starts with empty defaults when no config file is found (no error)
- [ ] Logs a clear error and exits when config file exists but is malformed
- [ ] `GE_PORT`, `GE_LOG_LEVEL`, `GE_LOG_FORMAT`, `GE_TLS`, `GE_TLS_CERT`, `GE_TLS_KEY` override config file values
- [ ] Unit tests for: valid config, missing file, malformed file, env var overrides, partial config

## Blocked by

None - can start immediately

## User stories addressed

- User story 1
- User story 4
- User story 5
- User story 11
- User story 13

Start by reading the current env.ts validation, dotenv loading, and CONFIGURATION_FOLDER_PATH logic, then compare the required config shape with parent issue #1660. Add unit tests for valid, missing, malformed, overridden, and partial configurations; done means the listed acceptance criteria pass, including clear failure on malformed files and empty defaults when files are absent.

Written by the indexing model from the issue text.