Deploy: support URI-form (postgresql://) connection strings — Npgsql's key-value-only parser rejects them

[mforce]

Deployment-readiness gap (#244), BLOCKER.

Problem

Many managed-Postgres platforms expose the connection as a libpq-style
postgresql://user:pass@host:5432/db URI. Npgsql's connection-string parser
accepts only key-value form (Host=…;Username=…;Password=…;Database=…) — a
postgresql:// URI throws at data-source construction. The value flows unmodified:
GetConnectionString("Default") (Program.cs:192) → builder.UseNpgsql(connectionString) (PostgresDbContextConfigurator.cs). A repo-wide search confirms there is no URI→key-value conversion anywhere. On any host that hands us a URI, the process throws at startup before it binds a port.

This is a portable robustness fix, not tied to one provider — URI-form connection strings are the norm across managed-Postgres platforms.

Fix (app-side)

Add a small postgresql:// → NpgsqlConnectionStringBuilder shim in
PostgresDbContextConfigurator (use System.Uri + the builder, not string
hacking — prefer framework facilities): if the configured connection string
parses as a postgres/postgresql URI, translate host / port / user / password /
database (and query params such as sslmode) into key-value form before
UseNpgsql; otherwise pass it through unchanged. Which platform env-var holds the
URI is deploy-repo config, not app code.

Verify

Set ConnectionStrings__Default=postgresql://u:p@h:5432/db and boot — reproduces
the throw today; after the fix, parses and connects. Key-value strings keep working
unchanged.

Part of the #244 deployment-readiness audit; tracked in epic #15.

[mforce]

Done — merged in PR #295. postgres:///postgresql:// URIs are translated to Npgsql key-value form before use (host, port default 5432, URL-decoded password, database from path, sslmode/cert query params mapped; common libpq params like channel_binding/target_session_attrs/application_name/connect_timeout mapped to their Npgsql keywords; genuinely-unknown params skip-with-warning rather than crashing the boot). Key-value strings pass through unchanged. Fully resolved in-repo.