Allow escaping special single-character wildcard "_" when doing partial matching

[DL6ER]

What does this implement/fix?

See title. Discussion ongoing in that issue ticket, hence, opening in draft mode here

If user use _ when matching domains or clients, this will not match a literal underscore but act as the SQLite3 default single-character wildcard. This PR allows users to escape underscores in domains/clients by escaping their underscore like \_. This was not possible before.

---

Related issue or feature (if applicable): Fixes #2549

Pull request in docs with documentation (if applicable): N/A

---

By submitting this pull request, I confirm the following:

1. I have read and understood the contributors guide, as well as this entire template. I understand which branch to base my commits and Pull Requests against.
2. I have commented my proposed changes within the code.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)

Checklist:

• The code change is tested and works locally.
• I based my code and PRs against the repositories development branch.
• I signed off all commits. Pi-hole enforces the DCO for all contributions
• I signed all my commits. Pi-hole requires signatures to verify authorship
• I have read the above and my PR is ready for review.

[DL6ER]

Related documentation on the web interface: pi-hole/web#3578

[rrobgill]

All working correctly both via API, and through the web interface.

Escaped underscore now searches correctly, and unescaped underscore remains as single character wildcard.

[Copilot]

Pull Request Overview

This PR enables escaping of underscore characters in domain and client name searches to prevent unintended wildcard matching. By default, SQLite treats _ as a single-character wildcard, but users can now escape underscores with \_ to match literal underscore characters.

• Add ESCAPE '\\' clause to SQL LIKE queries to enable backslash escaping
• Increase buffer allocation for like_name to accommodate escaped characters
• Replace sprintf with snprintf for safer string formatting

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/database/gravity-db.c	Add ESCAPE clause to LIKE queries and improve memory allocation/string formatting
src/api/queries.c	Add ESCAPE clause to domain and client search queries

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[DL6ER]

Rebased onto latest development. This reduces the number of commits from 5 to 3.