replace XTDB references with Datomic

- Update project overview in README.md and CLAUDE.md to reference Datomic
- Replace documentation links from XTDB to Datomic
- Update database layer descriptions from XTDB to Datomic
- Simplify configuration documentation (remove XTDB-specific connection details)
- Update data model to use Datomic entity/attribute naming conventions
- Update notes section to reference Datalog instead of XTQL
- Remove XTDB dependencies and test fixtures from test suite
- Simplify test utilities to work without XTDB test nodes

README.md

@@ -4,7 +4,7 @@ Backend application to store my code snippets and make them available via REST A
 
 [TUI CMS / MCP server companion project](https://git.travisshears.com/travisshears/snippets_cms)
 
-This project is written in [Clojure](https://clojure.org/) and data is stored in [XTDB](https://xtdb.dev/).
+This project is written in [Clojure](https://clojure.org/) and data is stored in [Datomic](https://www.datomic.com/).
 
 ## Links
 
@@ -12,7 +12,7 @@ This project is written in [Clojure](https://clojure.org/) and data is stored in
 - [basic web development clojure tutorial](https://clojure-doc.org/articles/tutorials/basic_web_development/)
 - [reitit malli coercion](https://cljdoc.org/d/metosin/reitit/0.9.1/doc/coercion/malli)
 - [malli docs](https://github.com/metosin/malli)
-- [xtdb docs](https://docs.xtdb.com/drivers/clojure/codox/xtdb.api.html#var-execute-tx)
+- [Datomic docs](https://docs.datomic.com/)
 
 ## Dev