Fix C4 annotation references for validation

Update C4 annotations to match declared element names and remove
invalid references to external systems not yet supported.

- Change "Data Models (app.models)" to "Data Validation Layer" to
  match component declaration
- Remove module path suffixes like "(app.database)" from references
- Remove "@c4-uses: SQLite Database" reference (external databases not
  yet supported by c4-literate-python)

These changes resolve all validation errors and allow the CI pipeline
to pass successfully.

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

Author: christosm-dev

app/database.py

@@ -69,9 +69,7 @@
 
 SYSTEM INTERACTIONS
 -------------------
-@c4-uses: SQLite Database - "Reads and writes note data" - "SQL/SQLite3"
-@c4-used-by: API Application (app.main) - "Requests database sessions" - "SQLAl
-          chemy ORM"
+@c4-used-by: API Application - "Requests database sessions" - "SQLAlchemy ORM"
 
 DATABASE SCHEMA
 ---------------

app/main.py

@@ -63,9 +63,9 @@
 
 SYSTEM INTERACTIONS
 -------------------
-@c4-uses: Database Layer (app.database) - "Manages note
+@c4-uses: Database Layer - "Manages note
           persistence" - "SQLAlchemy ORM"
-@c4-uses: Data Models (app.models) - "Validates request/response
+@c4-uses: Data Validation Layer - "Validates request/response
           data" - "Pydantic"
 @c4-used-by-person: API Consumer - "Creates, reads, and deletes
                     notes" - "HTTPS/JSON"

app/models.py

@@ -76,10 +76,8 @@
 
 SYSTEM INTERACTIONS
 -------------------
-@c4-used-by: API Application (app.main) - "Validates requests and formats respo
-          nses"
-@c4-uses: Database Layer (app.database) - "Serializes ORM objects to API respon
-          ses"
+@c4-used-by: API Application - "Validates requests and formats responses"
+@c4-uses: Database Layer - "Serializes ORM objects to API responses"
 
 ERROR HANDLING
 --------------