fix: hashtag timeline returns empty results

- inReplyTo stored as empty string "" for non-replies in timeline-store.js;
  filters using {$exists: false} missed these docs → hashtag/public timelines empty
- Fix: omit inReplyTo/quoteUrl fields entirely when empty (undefined spread)
- Fix: update all {$exists: false} filters to also match {inReplyTo: ""}
- Fix: hashtag timeline query uses case-insensitive prefix regex so
  "politics" matches "on/politics", "wandern" matches stored "wandern"
- DB migration: unset inReplyTo:"" on 620 existing ap_timeline documents

lib/mastodon/routes/accounts.js

@@ -439,7 +439,7 @@ router.get("/api/v1/accounts/:id/statuses", tokenRequired, scopeRequired("read",
       ];
     }
     if (req.query.exclude_replies === "true") {
-      baseFilter.inReplyTo = { $exists: false };
+      baseFilter.$or = [{ inReplyTo: { $exists: false } }, { inReplyTo: "" }];
     }
     if (req.query.exclude_reblogs === "true") {
       baseFilter.type = { $ne: "boost" };

lib/mastodon/routes/timelines.js

@@ -114,7 +114,7 @@ router.get("/api/v1/timelines/public", async (req, res, next) => {
     // Public timeline: only public visibility, no context items, no replies
     const baseFilter = {
       isContext: { $ne: true },
-      inReplyTo: { $exists: false },
+      $or: [{ inReplyTo: { $exists: false } }, { inReplyTo: "" }],
       visibility: "public",
     };
 
@@ -220,11 +220,18 @@ router.get("/api/v1/timelines/tag/:hashtag", async (req, res, next) => {
     const limit = parseLimit(req.query.limit);
     const hashtag = req.params.hashtag;
 
+    // Match the hashtag case-insensitively and as a prefix segment
+    // (e.g. "politics" matches "on/politics", "politics/foo").
+    // Escape to prevent regex injection — hashtag comes from a URL param.
+    const escapedHashtag = hashtag.replace(/[.*+?^${}()|[\]\\]/g, "\\$&");
+    const tagRegex = new RegExp(`(^|/)${escapedHashtag}(/|$)`, "i");
+
     const baseFilter = {
       isContext: { $ne: true },
-      inReplyTo: { $exists: false },
+      // inReplyTo is absent on non-replies; legacy docs may store "" — exclude both
+      $or: [{ inReplyTo: { $exists: false } }, { inReplyTo: "" }],
       visibility: { $in: ["public", "unlisted"] },
-      category: hashtag,
+      category: tagRegex,
     };
 
     const { filter, sort, reverse } = buildPaginationQuery(baseFilter, {

lib/timeline-store.js

@@ -348,11 +348,12 @@ export async function extractObjectData(object, options = {}) {
     // Attachment extraction failed — non-critical
   }
 
-  // In-reply-to — Fedify uses replyTargetId (non-fetching)
-  const inReplyTo = object.replyTargetId?.href || "";
+  // In-reply-to — Fedify uses replyTargetId (non-fetching).
+  // Only set when non-empty — $exists:false filters depend on field absence.
+  const inReplyTo = object.replyTargetId?.href || undefined;
 
   // Quote URL — Fedify reads quoteUrl / _misskey_quote / quoteUri
-  const quoteUrl = object.quoteUrl?.href || "";
+  const quoteUrl = object.quoteUrl?.href || undefined;
 
   // Interaction counts — not fetched at ingest time. The three collection
   // fetches (getReplies, getLikes, getShares) each trigger an HTTP round-trip
@@ -382,8 +383,8 @@ export async function extractObjectData(object, options = {}) {
     photo,
     video,
     audio,
-    inReplyTo,
-    quoteUrl,
+    ...(inReplyTo ? { inReplyTo } : {}),
+    ...(quoteUrl ? { quoteUrl } : {}),
     counts,
     pollOptions,
     votersCount,