Document to SourceFile link + cascade semantics

[AymanL]

Parent PRD

#33

What to build

Add a nullable FK from Document to SourceFile to make the metadata-only and file-attached states explicit, with the correct cascade semantics (see PRD §Implementation Decisions — Cascade deletion). No pipeline rewiring; models, migrations, and minimal admin only.

Acceptance criteria

• Document has a nullable source_file FK to SourceFile (on_delete=CASCADE so that deleting a SourceFile deletes the linked Document).
• A Document with source_file=None is valid (metadata-only state).
• A Document can be linked to at most one SourceFile.
• Migration is generated and applies cleanly.
• Tests: metadata-only Document (no SourceFile) is created successfully; deleting a SourceFile cascades and removes the linked Document; a Document cannot be linked to more than one SourceFile.

Blocked by

• Blocked by Document model + DocumentChunk FK + FileMetadata removal #44 (Document model must exist)

User stories addressed

Reference by number from the parent PRD:

• User story 4 (SourceFile represents only the stored file)
• User story 5 (Document links to at most one SourceFile)
• User story 6 (deleting SourceFile cascades to Document)