docs: fix terminology - replace "external" with "in-store"

Object storage is an integral part of DataJoint architecture,
not "external". Updated docstrings to use correct terminology:
- "external" → "in-store" or "store only"

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: dimitri-yatsenko

src/datajoint/builtin_codecs/__init__.py

@@ -7,13 +7,13 @@
 
 Built-in Codecs:
     - ``<blob>``: Serialize Python objects (in-table storage)
-    - ``<blob@>``: Serialize Python objects (external with hash-addressed dedup)
+    - ``<blob@>``: Serialize Python objects (in-store with hash-addressed dedup)
     - ``<attach>``: File attachment (in-table storage)
-    - ``<attach@>``: File attachment (external with hash-addressed dedup)
-    - ``<hash@>``: Hash-addressed storage with MD5 deduplication (external only)
-    - ``<object@>``: Schema-addressed storage for files/folders (external only)
-    - ``<npy@>``: Store numpy arrays as portable .npy files (external only)
-    - ``<filepath@store>``: Reference to existing file in store (external only)
+    - ``<attach@>``: File attachment (in-store with hash-addressed dedup)
+    - ``<hash@>``: Hash-addressed storage with MD5 deduplication (store only)
+    - ``<object@>``: Schema-addressed storage for files/folders (store only)
+    - ``<npy@>``: Store numpy arrays as portable .npy files (store only)
+    - ``<filepath@store>``: Reference to existing file in store (store only)
 
 Example - Creating a Custom Codec:
     Here's how to define your own codec, modeled after the built-in codecs::

src/datajoint/builtin_codecs/filepath.py

@@ -18,7 +18,7 @@ class FilepathCodec(Codec):
     exists in the storage backend. Unlike ``<attach>`` or ``<object@>``, no
     file copying occurs - only the path is recorded.
 
-    External only - requires @store.
+    Store only - requires @store.
 
     This codec gives users maximum freedom in organizing their files while
     reusing DataJoint's store configuration. Files can be placed anywhere
@@ -64,7 +64,7 @@ class Recordings(dj.Manual):
     name = "filepath"
 
     def get_dtype(self, is_store: bool) -> str:
-        """Filepath is external only."""
+        """Filepath requires a store (use @store syntax)."""
         if not is_store:
             raise DataJointError(
                 "<filepath> requires @ symbol. Use <filepath@> for default store or <filepath@store> to specify store."