Implement left/right voting on entries

GET /entry/{pid}/interactions/{mode} now reads the vote tally and, for
mode=left/right (auth required), toggles the user's vote: new vote, repeat
same mode removes it, different mode switches. Returns {left, right,
selected}. A UNIQUE(uid, pid) constraint on the vote table prevents
duplicate votes. The JS frontend renders Links/Rechts buttons with live
counts under each entry.

This completes the voting feature that was left commented-out and broken
in the Flask version.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

entry.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"database/sql"
 	"fmt"
 	"image"
 	"image/gif"
@@ -105,8 +106,64 @@ func handleCreateEntry(w http.ResponseWriter, r *http.Request) {
 	})
 }
 
+// handleInteractions liest den Abstimmungsstand eines Beitrags und schaltet bei
+// mode=left/right die Stimme des angemeldeten Nutzers um:
+//   - keine bisherige Stimme -> neue Stimme
+//   - gleiche Stimme erneut   -> Stimme zurückziehen (Toggle)
+//   - andere Stimme           -> auf den neuen Modus wechseln
+// mode=none liest nur (z. B. beim Laden des Feeds).
 func handleInteractions(w http.ResponseWriter, r *http.Request) {
-	writeError(w, http.StatusNotImplemented, "Voting ist noch nicht implementiert")
+	pid, err := strconv.ParseInt(chi.URLParam(r, "pid"), 10, 64)
+	if err != nil {
+		writeError(w, http.StatusBadRequest, "Ungültige pid")
+		return
+	}
+	mode := chi.URLParam(r, "mode")
+
+	var uid int64
+	if s, ok := getSession(r); ok {
+		uid = s.UID
+	}
+
+	if mode == "left" || mode == "right" {
+		if uid == 0 {
+			writeError(w, http.StatusUnauthorized, "Melde dich an, um abzustimmen!")
+			return
+		}
+
+		var prev string
+		err := db.QueryRow(`SELECT mode FROM vote WHERE uid = ? AND pid = ?`, uid, pid).Scan(&prev)
+		switch {
+		case err == sql.ErrNoRows:
+			db.Exec(`INSERT INTO vote (uid, pid, mode) VALUES (?, ?, ?)`, uid, pid, mode)
+		case err == nil && prev == mode:
+			db.Exec(`DELETE FROM vote WHERE uid = ? AND pid = ?`, uid, pid)
+		case err == nil:
+			db.Exec(`UPDATE vote SET mode = ? WHERE uid = ? AND pid = ?`, mode, uid, pid)
+		default:
+			writeError(w, http.StatusInternalServerError, "Datenbankfehler")
+			return
+		}
+	}
+
+	selected := "none"
+	if uid != 0 {
+		var cur string
+		if err := db.QueryRow(`SELECT mode FROM vote WHERE uid = ? AND pid = ?`, uid, pid).Scan(&cur); err == nil {
+			selected = cur
+		}
+	}
+
+	var left, right int
+	db.QueryRow(`SELECT COUNT(*) FROM vote WHERE pid = ? AND mode = 'left'`, pid).Scan(&left)
+	db.QueryRow(`SELECT COUNT(*) FROM vote WHERE pid = ? AND mode = 'right'`, pid).Scan(&right)
+
+	writeJSON(w, http.StatusOK, map[string]any{
+		"pid":      pid,
+		"left":     left,
+		"right":    right,
+		"selected": selected,
+	})
 }
 
 // storeImage skaliert ein hochgeladenes Bild auf max. 1024px und speichert es unter static/media.