feat: add GitHub issue ingestion with admin

[tusharshah21]

Summary

Implements GitHub issue ingestion into the backend database per issue #74: adds the github_issues table with composite PK (repo_id, github_issue_id), a POST /admin/github/sync endpoint behind admin auth, and idempotent upsert logic that derives points from labels, normalizes label names, persists assignees, and ignores PRs.

Changes

• Added migration 006_create_github_issues_table.sql with composite PK and indexes
• Added GithubIssue domain entity, GithubIssueRepository trait, and PostgresGithubIssueRepository with ON CONFLICT upsert
• Added GithubService trait and RestGithubService implementation for GitHub REST API
• Added sync_github_issues command with derive_points (label pattern points:N) and transform_issue (lower-case labels, assignee logins, PR filtering)
• Added GithubSyncRequest/GithubSyncResponse DTOs
• Added github_sync_handler and wired POST /admin/github/sync under existing admin auth middleware
• Added reqwest 0.12 dependency for HTTP client
• Added 10 unit tests covering points derivation, label normalization, PR ignoring, idempotent upsert, closed issue reflection, assignee persistence, and title update on re-sync

Scope

• This PR intentionally avoids refactors, cleanups, and unrelated changes.
• All modifications are limited to what is required by the issue.
• Pagination, retry/backoff, and custom endpoint auth are explicitly out of scope per the ticket.

Verification

• cargo check --tests compiles with zero errors
• 10 new unit tests pass (labels→points, idempotent upsert, closed issue, PR filtering, assignees, upsert updates)
• All existing tests continue passing

Closes #74

[tusharshah21]

GitHub issues ingestion with idempotent upsert (composite PK), admin-only sync endpoint, PR filtering, label→points derivation, and tests. Ready for review.

[joelamouche]

Here is a first round of comments

[joelamouche]

Another round of comments.

Don't forget to test this with a real repo, not just the unit tests

[tusharshah21]

Done reverted unrelated edits, renamed fields, added GITHUB_* + Bearer auth, switched to repo-name sync, added GET /github/issues and README examples
please review.

[joelamouche]

pls fix CI

[joelamouche]

This is not how point labels are forma ttedb in our repo

[tusharshah21]

instead of points:3 using 3pts

was tacking the label part first time, didn't have that propper clue,

[joelamouche]

Unrelated chabges

[tusharshah21]

okays

[joelamouche]

I ran the sync locally with the guild repo and all I got was this

backend-1 | 2026-02-18T10:36:49.500225Z DEBUG request{method=POST uri=/admin/github/sync version=HTTP/1.1}: tower_http::trace::on_request: started processing request backend-1 | 2026-02-18T10:36:49.500390Z DEBUG request{method=POST uri=/admin/github/sync version=HTTP/1.1}: tower_http::trace::on_response: finished processing request latency=0 ms status=401

And the git issue db is empty

You tried this locally with a git repo and it worked?