Feat v0.8.3 vector column type (#70)

Add vector(N) column type to db_tables manifests with three-tier
progressive enhancement: native pgvector on Postgres, JSONB fallback
without pgvector, TEXT fallback on SQLite. New db.query_similar()
Starlark builtin with dual-path dispatch.

- parseVectorDim validates 1..4096 dimensions
- mapColType gains hasPgvector parameter for tier selection
- HNSW index auto-created on pgvector backends
- starlarkToGoValue extended with list→JSON serialization
- cosineDistance helper for Go-side fallback computation
- ExtensionHandler gains SetCapabilities for install-time DDL
- Roadmap updated: v0.8.4 docs refresh + surface sizing fix

13 new tests (5 schema + 8 db module), all passing with -race.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/handlers/extensions.go

@@ -17,13 +17,19 @@ import (
 
 // ExtensionHandler serves extension management endpoints.
 type ExtensionHandler struct {
-	stores store.Stores
+	stores       store.Stores
+	capabilities map[string]bool
 }
 
 func NewExtensionHandler(stores store.Stores) *ExtensionHandler {
 	return &ExtensionHandler{stores: stores}
 }
 
+// SetCapabilities stores the detected environment capabilities map.
+func (h *ExtensionHandler) SetCapabilities(caps map[string]bool) {
+	h.capabilities = caps
+}
+
 // validTiers is the set of accepted extension tier values.
 var validTiers = map[string]bool{
 	models.ExtTierBrowser:  true,
@@ -231,7 +237,7 @@ func (h *ExtensionHandler) AdminInstallExtension(c *gin.Context) {
 	SyncManifestPermissions(c, h.stores, pkg.ID, manifestMap)
 
 	if tables, ok := ParseDBTables(manifestMap); ok {
-		if err := CreateExtTables(c.Request.Context(), database.DB, database.IsPostgres(), h.stores, pkg.ID, tables); err != nil {
+		if err := CreateExtTables(c.Request.Context(), database.DB, database.IsPostgres(), h.stores, pkg.ID, tables, h.capabilities["pgvector"]); err != nil {
 			log.Printf("[ext-db] schema create failed for %s: %v", pkg.ID, err)
 		}
 	}