refactor: workspace secrets-core + secrets-mcp MCP SaaS

- Split library (db/crypto/service) and MCP/Web/OAuth binary
- Add deploy examples and CI/docs updates

Made-with: Cursor

crates/secrets-core/src/config.rs

@@ -0,0 +1,20 @@
+use anyhow::Result;
+
+/// Resolve database URL from environment.
+/// Priority: `SECRETS_DATABASE_URL` env var → error.
+pub fn resolve_db_url(override_url: &str) -> Result<String> {
+    if !override_url.is_empty() {
+        return Ok(override_url.to_string());
+    }
+
+    if let Ok(url) = std::env::var("SECRETS_DATABASE_URL")
+        && !url.is_empty()
+    {
+        return Ok(url);
+    }
+
+    anyhow::bail!(
+        "Database not configured. Set the SECRETS_DATABASE_URL environment variable.\n\
+         Example: SECRETS_DATABASE_URL=postgres://user:pass@host:port/dbname"
+    )
+}