feat(wordpress): only assign tags that are established

_resolve_wp_tag_ids created a WordPress tag for every keyword the rewriter
invented, up to 12 per post. That is where the 3.055 tags for 955 posts
came from - 1.683 of them used exactly once, 473 attached to no post at
all. The categories are stable now, but the tags would simply grow back.

Three changes, all on the write path:

A proposed tag has to appear for wordpress_new_tag_min_proposals (3)
different articles before it is created. Proposals are counted in the new
tag_proposals table, keyed on (name, article), so re-publishing an article
does not inflate its own count. Tags that already exist in WordPress are
assigned as before - the gate only guards creation.

Only the first wordpress_max_tags_per_post (5) tags reach WordPress. The
full list still feeds the category rules, which were validated against it.

The lookup fallback of reusing the first search hit is gone. It filed
"Camping" under the unrelated existing tag "Campingplatz" whenever the
exact tag was missing, which quietly produced wrong tags rather than none.

If the proposal bookkeeping fails, nothing is creatable that run: existing
tags still get assigned and the taxonomy stays put, rather than falling
back to creating everything.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/app/db.py

@@ -118,6 +118,21 @@ def init_db() -> None:
                 UNIQUE(source_url)
             );
 
+            -- One row per (tag, article) the rewriter ever proposed, whether or
+            -- not the tag made it into WordPress. The number of rows per
+            -- name_key is the "is this tag established" signal that gates
+            -- creating the tag in WordPress.
+            CREATE TABLE IF NOT EXISTS tag_proposals (
+                id INTEGER PRIMARY KEY AUTOINCREMENT,
+                name_key TEXT NOT NULL,
+                name TEXT NOT NULL,
+                article_id INTEGER NOT NULL DEFAULT 0,
+                created_at TEXT NOT NULL DEFAULT (datetime('now')),
+                UNIQUE(name_key, article_id)
+            );
+
+            CREATE INDEX IF NOT EXISTS idx_tag_proposals_name_key ON tag_proposals(name_key);
+
             CREATE INDEX IF NOT EXISTS idx_articles_source_article_id ON articles(source_article_id);
             CREATE INDEX IF NOT EXISTS idx_articles_source_hash ON articles(source_hash);
             CREATE UNIQUE INDEX IF NOT EXISTS uq_articles_feed_source_article_id