fix(storage-postgres): build socket-aware connection URLs (#52)

[flightlesstux]

What

Fix extenddb init --pg-host <socket-path> producing a connection URL that the daemon's PostgreSQL driver rejects at startup with error with configuration: empty host.

Why

Closes #52

init was writing URLs of the form postgresql://user:pw@/var/run/postgresql:5432/db into both extenddb.toml and the catalog data_database_connection_string row. The unencoded forward slashes in the host segment break standard URL parsing, so sqlx::PgPool::connect rejected the URL at daemon startup. Init succeeded, serve failed, and users had to hand-edit both the TOML and the catalog row to recover.

Changes

• crates/storage-postgres/src/config.rs: new build_connection_url() helper. When host starts with / it emits the documented PostgreSQL socket form postgresql://user:pw@localhost:5432/db?host=/var/run/postgresql. TCP hosts keep the classic form. parse_connection_string() now honors a ?host=<path> query parameter with last-value-wins libpq semantics; an empty value falls back to the URL host so existing parse callers stay backwards compatible.
• crates/storage-postgres/src/bootstrapper.rs: app_connection_url() delegates to the new builder. No other behavior change.

Testing done

Added 10 unit tests in crates/storage-postgres/src/config.rs:

• TCP and socket URL build shapes.
• TCP URL parse path unchanged.
• Socket URL parse extracts the ?host= value.
• Empty ?host= falls back to the URL host.
• Unknown query parameters are ignored.
• Round-trip through the custom parser for both TCP and socket URLs.
• Round-trip through sqlx::postgres::PgConnectOptions::from_str for both URL shapes. This is the same parser the daemon uses, so it directly verifies the fix at the failure layer.

Results:

```
cargo test --workspace --lib 382/382 passed
cargo clippy -p extenddb-storage-postgres --all-targets -- -D warnings clean
cargo fmt -p extenddb-storage-postgres -- --check (on the two changed files) clean
```

Full `cargo test --workspace` (including pytest/external integration suites) was not exercised in this branch because it requires a running PostgreSQL instance and provisioned test credentials.

Checklist

• I have read CONTRIBUTING.md
• All workspace lib tests pass (`cargo test --workspace --lib`)
• Changed files are formatted (`cargo fmt`)
• Clippy is clean on the modified crate (`-D warnings`)
• Added unit tests for new functionality
• No documentation changes needed (URL form change is internal; observable behavior is identical for TCP setups and now actually works for socket setups)
• No breaking changes

[jcshepherd]

Hi Ercan! Thanks for the PR. I'll let the reviewers weigh in on this approach vs #92 .