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>

db.go

@@ -40,7 +40,8 @@ CREATE TABLE IF NOT EXISTS vote (
     id   INTEGER PRIMARY KEY AUTOINCREMENT,
     uid  INTEGER,
     pid  INTEGER,
-    mode TEXT
+    mode TEXT,
+    UNIQUE(uid, pid)
 );
 `