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.

app/controllers/packages_controller.rb

@@ -362,7 +362,7 @@ class PackagesController < ApplicationController
     # text search
     if params[:search].present?
       logger.debug "Searching for #{params[:search]}"
-      packages = packages.general_search(params[:search])
+      packages = search_packages(params[:search])
     end
 
     case params[:show]
@@ -378,6 +378,51 @@ class PackagesController < ApplicationController
     packages
   end
 
+  # Text search for packages: an exact package name match wins without
+  # touching the vector service. Otherwise packages whose name starts
+  # with the query (or contains all query tokens, catching compound
+  # words like "sqlite browser" -> "sqlitebrowser") are promoted above
+  # the semantic (vector) nearest neighbor results. Classic PostgreSQL
+  # full-text search kicks in when the vector service cannot be reached.
+  MAX_PROMOTED_NAME_MATCHES = 8
+  MAX_SEMANTIC_RESULTS = 100
+  private_constant :MAX_PROMOTED_NAME_MATCHES, :MAX_SEMANTIC_RESULTS
+
+  def search_packages(query)
+    exact_match = Package.find_by(name: query)
+    return Package.where(id: exact_match.id) if exact_match
+
+    tokens = query.to_s.split(/\s+/)
+    name_matches = (
+      Package.name_starts_with(query).limit(MAX_PROMOTED_NAME_MATCHES) +
+      Package.name_contains_all(tokens).limit(MAX_PROMOTED_NAME_MATCHES)
+    ).uniq(&:id)
+
+    results = (name_matches + semantic_results(query)).uniq(&:id)
+
+    # Queries that share no vocabulary with our package data can produce
+    # no meaningful results. Skip the vector service round-trip and show
+    # an honest empty result instead of random packages.
+    results.empty? ? Package.none : results
+  end
+
+  # Semantic (vector) nearest neighbor search with a full-text fallback
+  # for when the external embedding service is unreachable.
+  def semantic_results(query)
+    unless Package.lexical_overlap?(query)
+      logger.debug "Query '#{query}' has no lexical overlap with package data"
+      return []
+    end
+
+    begin
+      Package.nearest_to_text(query, limit: MAX_SEMANTIC_RESULTS).to_a
+    rescue Vectorizer::Error => e
+      logger.error "Semantic search failed: #{e.message}"
+      flash.now['error'] = 'Semantic search is unavailable right now. Showing full-text matches.'
+      Package.general_search(query).limit(100).to_a
+    end
+  end
+
   # Store a random identifier and the client's IP address in the session
   # for later identification.
   # def create_user_token