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/validators/registries/pypi.go

@@ -6,6 +6,7 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"net/url"
 	"strings"
 	"time"
 
@@ -52,8 +53,13 @@ func ValidatePyPI(ctx context.Context, pkg model.Package, serverName string) err
 
 	client := &http.Client{Timeout: 10 * time.Second}
 
-	url := fmt.Sprintf("%s/pypi/%s/%s/json", pkg.RegistryBaseURL, pkg.Identifier, pkg.Version)
-	req, err := http.NewRequestWithContext(ctx, http.MethodGet, url, nil)
+	// PathEscape so an identifier that smuggles "/" or ".." cannot redirect
+	// the metadata fetch to a different package than the one being claimed.
+	fetchURL := fmt.Sprintf("%s/pypi/%s/%s/json",
+		pkg.RegistryBaseURL,
+		url.PathEscape(pkg.Identifier),
+		url.PathEscape(pkg.Version))
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, fetchURL, nil)
 	if err != nil {
 		return fmt.Errorf("failed to create request: %w", err)
 	}