Limit OG image generation concurrency (#47)

* Limit OG image generation to 2 concurrent goroutines

Under heavy crawler traffic, unbounded OG generation goroutines were
constantly contesting the SQLite write lock, starving the pipeline.
Non-blocking semaphore skips generation when at capacity — the image
will be generated on the next visit instead.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Move OG semaphore to call site to prevent goroutine churn

Acquiring the semaphore before spawning the goroutine ensures excess
requests under crawler spikes don't create unbounded goroutines that
immediately exit.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: retlehs

internal/http/handlers.go

@@ -29,6 +29,10 @@ import (
 
 const perPage = 12
 
+// ogSem limits concurrent OG image generation to avoid starving the pipeline
+// of DB write access under heavy crawler traffic.
+var ogSem = make(chan struct{}, 2)
+
 // captureError reports a non-panic error to Sentry with the request's hub.
 // It silently ignores context cancellation errors (timeouts, client disconnects)
 // since these are expected during normal operation.
@@ -223,8 +227,15 @@ func handleDetail(a *app.App, tmpl *templateSet) http.HandlerFunc {
 
 		ogKey := "social/" + pkg.Type + "/" + pkg.Name + ".png"
 		if pkg.OGImageGeneratedAt == nil {
-			// Generate on demand in background
-			go generatePackageOG(a, pkg)
+			// Generate on demand in background (skip if at capacity)
+			select {
+			case ogSem <- struct{}{}:
+				go func() {
+					defer func() { <-ogSem }()
+					generatePackageOG(a, pkg)
+				}()
+			default:
+			}
 		}
 
 		displayName := pkg.Name