Fix SSRF, XSS, dedup, force-post, and DB hot-path issues

- Add internal/safehttp: hardened HTTP client (DNS-resolved dial guard
  blocking loopback/RFC1918/CGNAT/link-local, redirect re-validation,
  body-size cap) and rewire article/feed/thumb clients through it
- Cap goquery body at 5 MiB so a hostile origin can't OOM the process
- search.js: reject non-http(s) hrefs to block stored XSS via javascript:
- dedup: tracking-param key "CMP" was unreachable (lookup lowercases);
  fixed to "cmp" so CMP= is actually stripped from canonical URLs
- ForcePost: postItem now returns bool; on dedup-skip ForcePost returns
  false so !post falls back to DB lookup instead of silently consuming
- Bound reaction callbacks behind an 8-slot semaphore; drop overflow
- Add stories indexes on (channel, classified, seen_at DESC),
  (classified, seen_at DESC), and partial image_url to kill full scans
  in IsKnownImageURL and ORDER BY seen_at hot paths
- Surface FTS5 probe Scan error instead of swallowing it

internal/web/thumbs.go

@@ -23,6 +23,7 @@ import (
 	"golang.org/x/image/draw"
 	_ "golang.org/x/image/webp"
 
+	"pete/internal/safehttp"
 	"pete/internal/storage"
 )
 
@@ -36,7 +37,7 @@ const (
 )
 
 var (
-	thumbClient = &http.Client{Timeout: thumbFetchTimeout}
+	thumbClient = safehttp.NewClient(thumbFetchTimeout)
 
 	thumbMu       sync.Mutex
 	thumbInFlight = map[string]*sync.WaitGroup{}