docs: Reshape Endpoints & APIs concepts, archive deprecated streaming

[developerjamiu]

Phase 2 IA, PR 2 of 7 (tracked in #671): reshapes the Endpoints & APIs pages and moves the deprecated streaming API to the archive.

What changed

• Error handling & exceptions: added a "Handling errors on the client" section (the ServerpodClientException hierarchy and the -1 connection-error case) and a "Don't leak sensitive data" section. Corrected the client exception name from ServerpodException to ServerpodClientException.
• Streaming: added a worked example (live updates for a filtered query, using a streaming method plus a per-room server-events channel). Moved the deprecated "Streaming Endpoints" section to the archive, fully preserved with a banner.
• Working with endpoints: added "Point the client at each environment" (choosing the client URL per build with --dart-define).
• Endpoint middleware: renamed from "Middleware" to disambiguate from web-server middleware, with a cross-link. They are different APIs (pod.server.addMiddleware versus pod.webServer.addMiddleware).
• Configure HTTP calls: completed the conditional-import stub sample.
• Endpoint inheritance and File uploads: conformance, plus a fix (config/password.yaml to config/passwords.yaml).

Verification

Checked against the framework source: the client exception hierarchy (serverpod_client_exception.dart), createStream and postMessage (session.dart), and the two distinct middleware APIs. The --dart-define client-URL pattern is standard Flutter, and the conditional-import stub mirrors the page's existing platform-native example.

Notes

• Folds in the client-URL half of Document client URL configuration per environment and handling expired auth #654; the expired-auth half is handled in the Authentication PR.
• Structural changes (grouping, section rename, redirects) are deferred to the final PR of this series, so this PR keeps every page at its current URL.

Closes #648.

[Zfinix]

Approved, non-blocking