[feature] Add from: search operator and account_id query param

[VyrCossont]

Description

This pull request implements Mastodon-compatible functionality for restricting a search to statuses authored by a specific account:

• account_id parameter to search API
• from:localuser or from:remoteuser@domain as an additional (or only) term in search query text, useful for the majority of clients which don't support account_id (also implemented in Mastodon, documented only in online help)

There are two main use cases:

• Find your most recent conversations with somebody else
• Find your own posts

Checklist

• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have not leveraged AI to create the proposed changes.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[tsmethurst]

Looks deece! We should document this in the user guide somewhere. Would you mind creating a page in there and adding to mkdocs.yaml or whatever it is?

[VyrCossont]

@tsmethurst no problem. I found the docs preview workflow in CONTRIBUTING.md and will add something to the user guide.

[tsmethurst]

Thank you!