refactor(store,tools): migrate IDs from UUID to bigserial int64

All internal entity lookups now use bigserial primary keys (int64) while
GUIDs are retained for external/public identification. Updated store
functions (TouchProject, UpdateThoughtMetadata, AddThoughtAttachment) to
query by id instead of guid, added GetThoughtByID, changed semanticSearch
and all tool helpers to use *int64 project IDs, and updated retry/backfill
workers to use int64 thought IDs throughout.

internal/types/project.go

@@ -9,11 +9,12 @@ import (
 type ThoughtPatch struct {
 	Content  *string         `json:"content,omitempty"`
 	Metadata ThoughtMetadata `json:"metadata,omitempty"`
-	Project  *uuid.UUID      `json:"project_id,omitempty"`
+	Project  *int64          `json:"project_id,omitempty"`
 }
 
 type Project struct {
 	ID           uuid.UUID `json:"id"`
+	NumericID    int64     `json:"-"`
 	Name         string    `json:"name"`
 	Description  string    `json:"description,omitempty"`
 	CreatedAt    time.Time `json:"created_at"`