Add semantic package search via pgvector embeddings

Package text search now combines several strategies: exact name match,
name prefix promotion, compound word splitting ("sqlite browser" finds
"sqlitebrowser") and semantic nearest neighbor search over description
embeddings computed by an external embedding service (all-MiniLM-L6-v2,
384 dims) stored with the pgvector extension. Classic PostgreSQL
full-text search remains as fallback when the vector service is
unreachable. Gibberish queries without lexical overlap with the package
data return empty results instead of random matches.

Embeddings can be backfilled with bin/rails debshots:compute_vectors.

Also drops the unused lograge gem from the Gemfile.

config/initializers/will_paginate.rb

@@ -0,0 +1,6 @@
+# frozen_string_literal: true
+
+# Enable will_paginate's Array#paginate so that merged search results
+# (name prefix matches + semantic results) can be paginated like
+# ActiveRecord relations.
+require 'will_paginate/array'