Clarify entity.json vs practice.json docs, including per-practice path overrides

Practice.json already supported overriding insurance_path/pdf_path/patient_path
via Merge-Config, but the README and example only mentioned login overrides —
document that practices can have different folder paths, not just credentials.

README.md

@@ -49,7 +49,32 @@ Entities\
 
 ---
 
-## Setting up a new entity
+## The two config files
+
+There are two JSON files, and they work together — one sets the defaults for
+the whole entity, the other overrides just what's different for one practice.
+
+- **`entity.json`** — lives in the entity folder (e.g. `Entities\CAMBS\entity.json`).
+  Required. This is the default server info used by every practice under that
+  entity: which server, which login, which folder paths.
+- **`practice.json`** — lives inside one practice's folder (e.g.
+  `Entities\CAMBS\Export1\AJMAT\practice.json`). Optional — only add it if a
+  practice needs something *different* from `entity.json`. Whatever fields you
+  put in here replace the matching field from `entity.json` for that practice
+  only; anything you leave out still comes from `entity.json`.
+
+Common reasons to add a `practice.json`:
+- The practice has its own separate FTP/SFTP login instead of the shared one
+- The practice's files need to go to a **different folder path** on the
+  server than the standard `{practice}` pattern (e.g. the imaging vendor set
+  it up with a different folder name, or a nonstandard structure) — override
+  `insurance_path`, `pdf_path`, and/or `patient_path` to whatever that
+  practice actually needs
+
+You can mix and match — e.g. override just `pdf_path` for one practice and
+leave its login and every other path alone.
+
+### `entity.json`
 
 1. Create a folder under `Entities\` with the entity name (e.g. `Entities\CAMBS\`)
 2. Copy `run.bat` into it
@@ -71,15 +96,41 @@ Entities\
 
 4. Create the `Export1\` folder and a subfolder for each practice inside the entity folder
 
+`{practice}` in any path gets automatically replaced with that practice's
+folder name — so one `entity.json` can serve every practice as long as they
+all follow the same folder-naming pattern on the server.
+
 ---
 
-## Setting up a practice with its own FTP login
+## `practice.json` — overriding one practice
 
-If one practice has its own FTP username/password (different from the entity default):
+If one practice needs a different login, a different folder path, or both:
 
 1. Create a `practice.json` file inside that practice's folder
 2. Copy from `Entities\EXAMPLE\Export1\PRACTICE_NAME\practice.json`
-3. Fill in only the username and password that differ — everything else comes from entity.json
+3. Delete every line you *don't* need to change — keep only the fields that
+   differ for this practice. Everything you delete falls back to `entity.json`.
+
+Example — this practice only has its own PDF folder name, nothing else differs:
+
+```json
+{
+  "ftps": {
+    "pdf_path": "/{practice}/PDF_Statements_Custom"
+  }
+}
+```
+
+Example — this practice has its own SFTP login but uses the same paths as everyone else:
+
+```json
+{
+  "sftp": {
+    "username": "ajmat-sftp-user",
+    "password": "ajmat-sftp-pass"
+  }
+}
+```
 
 ---