security: fix open redirect and add small hardening (#1227)

## What this does

Fixes an open redirect plus a handful of small, mostly-invisible
hardening changes around it.

## What's in it

- **The fix**: trailing-slash redirects no longer let `//evil.com/`
redirect off-host.
- **SSRF block** on the DNS/HTTP namespace verification — refuses to
dial loopback, RFC1918, link-local, multicast, CGNAT, or
IP-literal/single-label \"domains\".
- **Slow-client guard**: `ReadTimeout` and `IdleTimeout` on the HTTP
server (no `WriteTimeout` — would risk cutting off legitimate
multi-package publishes).
- **Defence-in-depth headers** on the UI (`/`) and 404 responses.
- **5xx responses** stop leaking raw DB error text to clients (logged
server-side instead).
- **Search filter** stops treating `%` and `_` as wildcards.
- **Validators**: PyPI/NuGet identifiers URL-escaped before fetching;
MCPB no longer follows redirects.
- **Misc**: `Version` field bounded to 255 chars; logs quote user input;
`BlockedNamespaces` denylist now catches subdomain claimants.

Each change is its own commit so they can be reviewed (or reverted)
independently.

## Test plan

- [ ] CI green
- [ ] Synthetic publish in staging for each package type (npm, pypi,
nuget, oci, mcpb) — confirms validator changes don't regress
- [ ] Quick browser check of `/` after deploy to confirm UI still loads
with the new CSP
- [ ] `curl https://<host>//evil.com/` — confirm the open redirect is
closed

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/api/handlers/v0/edit.go

@@ -3,6 +3,7 @@ package v0
 import (
 	"context"
 	"errors"
+	"log"
 	"net/http"
 	"net/url"
 	"strings"
@@ -74,7 +75,8 @@ func RegisterEditEndpoints(api huma.API, pathPrefix string, registry service.Reg
 			if errors.Is(err, database.ErrNotFound) {
 				return nil, huma.Error404NotFound("Server not found")
 			}
-			return nil, huma.Error500InternalServerError("Failed to get current server", err)
+			log.Printf("edit: get current server (%q/%q) failed: %v", serverName, version, err)
+			return nil, huma.Error500InternalServerError("Failed to get current server")
 		}
 
 		// Verify edit permissions for this server using the existing server name