feat: let jev answer the crawler's classification, and keep its confidence

The crawler asked gpt-4.1-mini one three-way question per undecided page, and
then threw away the confidence it asked for in the same prompt. Jev answers that
question as a typed choice for $0.042 per million in and nothing out, with a
confidence that falls out of the distribution rather than the model's opinion of
itself.

It cannot write learnedSignals though -- rule_value is free text -- so the
expensive model is still what teaches a new site. Once a site has banked enough
rules and jev is sure, we stop paying for it. A dead signal call now falls back
to the jev answer instead of losing the page.

confidence is stored on crawler_page_classifications, additively, in both
dialects. Pattern, rule and signal-model rows keep a null rather than borrowing
a number that belonged to a different answer.

The openrouter slug is unverified -- jev is beta there and absent from
/api/v1/models -- so both the slug and the endpoint are env overridable, and a
bad slug degrades to the old path instead of breaking the crawl.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EZ6bEoR6m6vrksSYRiXwVD

src/db/postgres.js

@@ -182,6 +182,7 @@ exec(`
     url TEXT PRIMARY KEY,
     site_name TEXT NOT NULL,
     classification TEXT NOT NULL,
+    confidence DOUBLE PRECISION,
     pattern TEXT,
     classified_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
   )
@@ -223,4 +224,12 @@ exec(`
   )
 `);
 
+// same story as the sqlite side: the creates above skip a database that already
+// has the table, so additions land here too. postgres does have the IF NOT EXISTS
+// form so this one stays boring. Additive only.
+exec(`
+  ALTER TABLE crawler_page_classifications
+  ADD COLUMN IF NOT EXISTS confidence DOUBLE PRECISION
+`);
+
 module.exports = pgDb;