Add optional Xquik tweet source context

[kriptoburak]

Summary:

• Add an optional Xquik Topic field to the Twitter automation form so AI tweet prompts can include short Xquik tweet-search source examples when XQUIK_API_KEY is configured.
• Add xquik_utils.py to fetch bounded source examples from https://xquik.com/api/v1/x/tweets/search.
• Document the optional XQUIK_API_KEY setting.

Target improvements:

• Avoid scraping an empty URL before checking whether URL context was requested.
• Validate post interval values and reject negative intervals before scheduling.
• Validate malformed schedule times and stop saving passed scheduled times as scheduled tweet records.
• Reject empty tweet content before posting or storing the job.

Validation:

• python3 -m py_compile app.py xquik_utils.py
• git diff --check