Add granular 'obsidian' EntrySource to distinguish plugin-imported content from other sources

[chillu]

Currently, all content imported via the Obsidian plugin is marked with EntrySource = "computer", which is also used for other types of imports (such as desktop client or manual ingestion). This lack of granularity in the source attribution means:

• It's impossible to specifically target only Obsidian-imported content for actions (e.g. bulk deletion) from the UI or API;
• All "computer" entries are lumped together, and deleting Obsidian content inadvertently affects unrelated entries.

For example, in src/khoj/routers/api_content.py, the EntrySource type only allows "computer", "github", or "notion", which are saved as DBEntry.file_source in the database. There is no way to distinguish between Obsidian-originated content and other sources with the current model.

Proposed Solution:

• Add a new EntrySource value (e.g. "obsidian") in both frontend and backend to mark entries originating from the Obsidian plugin.
• Update validation, ingestion, and query logic to recognize and persist "obsidian" as a valid source.
• Adapt deletion, filtering, and API/listing features to allow distinguishing content by its specific EntrySource.

Motivation/Use-case:

• Safely perform operations (delete, filter, query, list) that affect only Obsidian-imported content.
• Prevent accidental data loss when deleting or modifying content ingested by other pathways.
• Enable better tracking and debugging of content origins.

[debanjum]

Good idea. We should implement this