feat(ai): pgvector embedding infrastructure for semantic search

Semantic-search slice 1 (infra). Adds the pgvector plumbing the backfill worker and
hybrid search will use:
- swap the Postgres image to pgvector/pgvector:pg16 (drop-in for pg16 data)
- Pgvector + Pgvector.EntityFrameworkCore (0.2.0, EF8-compatible); UseVector() on the
  runtime + design-time contexts
- Email.Embedding vector(768) column (nomic-embed-text dims), nullable, with an HNSW
  cosine index; ignored under the InMemory test provider
- migration: CREATE EXTENSION vector + column + HNSW index

Verified against a real pgvector container: extension, HNSW, and cosine search work, and
the full EF round-trip (store a Pgvector.Vector, CosineDistance operator, nearest-first
ordering) applies all migrations and passes. No vulnerable packages. Build + all 41 tests
pass. Column stays null until Ollama generates embeddings (search falls back to lexical).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/InboxIntel.Infrastructure/Migrations/AppDbContextModelSnapshot.cs

@@ -6,6 +6,7 @@ using Microsoft.EntityFrameworkCore.Infrastructure;
 using Microsoft.EntityFrameworkCore.Storage.ValueConversion;
 using Npgsql.EntityFrameworkCore.PostgreSQL.Metadata;
 using NpgsqlTypes;
+using Pgvector;
 
 #nullable disable
 
@@ -22,6 +23,7 @@ namespace InboxIntel.Infrastructure.Migrations
                 .HasAnnotation("Relational:MaxIdentifierLength", 63);
 
             NpgsqlModelBuilderExtensions.HasPostgresExtension(modelBuilder, "pg_trgm");
+            NpgsqlModelBuilderExtensions.HasPostgresExtension(modelBuilder, "vector");
             NpgsqlModelBuilderExtensions.UseIdentityByDefaultColumns(modelBuilder);
 
             modelBuilder.Entity("InboxIntel.Domain.Entities.AnalyticsAggregate", b =>
@@ -125,6 +127,9 @@ namespace InboxIntel.Infrastructure.Migrations
                     b.Property<DateTimeOffset>("CreatedAtUtc")
                         .HasColumnType("timestamp with time zone");
 
+                    b.Property<Vector>("Embedding")
+                        .HasColumnType("vector(768)");
+
                     b.Property<string>("GmailMessageId")
                         .IsRequired()
                         .HasMaxLength(64)
@@ -194,6 +199,11 @@ namespace InboxIntel.Infrastructure.Migrations
 
                     b.HasKey("Id");
 
+                    b.HasIndex("Embedding");
+
+                    NpgsqlIndexBuilderExtensions.HasMethod(b.HasIndex("Embedding"), "hnsw");
+                    NpgsqlIndexBuilderExtensions.HasOperators(b.HasIndex("Embedding"), new[] { "vector_cosine_ops" });
+
                     b.HasIndex("SearchVector");
 
                     NpgsqlIndexBuilderExtensions.HasMethod(b.HasIndex("SearchVector"), "GIN");