feat: add configurable remote engine API URL for embedding requests

[neotty]

This merge request makes the remote embedding API endpoint configurable at build time, instead of being hardcoded in source.

What changed

• Added a new Makefile variable: REMOTE_ENGINE_API_URL
• Wired REMOTE_ENGINE_API_URL into compilation defines as DBMEM_REMOTE_API_URL
• Updated remote embedding code to use DBMEM_REMOTE_API_URL
• Kept a safe default endpoint when no override is provided
• Updated build help and vars output to expose the new configuration value

How to use

• Default build:
  make
• Override endpoint:
  make REMOTE_ENGINE_API_URL=https://your-endpoint/v1/embeddings
• Remote-only build with override:
  make remote REMOTE_ENGINE_API_URL=https://your-endpoint/v1/embeddings

Validation

• Verified Makefile variables output includes REMOTE_ENGINE_API_URL
• Verified overridden REMOTE_ENGINE_API_URL is propagated into compiler defines
• No new diagnostics reported in modified files

[Copilot]

Pull request overview

This PR makes the remote embedding endpoint configurable at build time by introducing a Makefile variable that is propagated into the C compilation defines, replacing the previously hardcoded URL in the remote embedding implementation.

Changes:

• Added REMOTE_ENGINE_API_URL Makefile variable (with a default) and exposed it in help/vars output.
• Propagated REMOTE_ENGINE_API_URL into compilation defines as DBMEM_REMOTE_API_URL.
• Updated the remote embedding code to use DBMEM_REMOTE_API_URL with an in-source fallback default.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/dbmem-rembed.c	Uses DBMEM_REMOTE_API_URL (with a fallback default) instead of a hardcoded endpoint for remote embedding requests.
Makefile	Adds REMOTE_ENGINE_API_URL, prints it in help/vars, and passes it to the compiler as -DDBMEM_REMOTE_API_URL=....

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[Copilot]

This line starts with a tab character while in a top-level ifeq block (not inside a recipe). In GNU Make, a leading tab is treated as a recipe line and will error with “recipe commences before first target”. Remove the leading tab and align indentation with the surrounding variable assignments (spaces only).

Suggested change

	override DEFINES += -DDBMEM_REMOTE_API_URL=\"$(REMOTE_ENGINE_API_URL)\"
	override DEFINES += -DDBMEM_REMOTE_API_URL=\"$(REMOTE_ENGINE_API_URL)\"