feat(coderd): refactors github pr sync functionality (#22715)

- Adds `_API_BASE_URL` to `CODER_EXTERNAL_AUTH_CONFIG_`
- Extracts and refactors existing GitHub PR sync logic to new packages
`coderd/gitsync` and `coderd/externalauth/gitprovider`
- Associated wiring and tests

Created using Opus 4.6

coderd/database/queries/chats.sql

@@ -448,3 +448,52 @@ LIMIT
 
 -- name: GetChatByIDForUpdate :one
 SELECT * FROM chats WHERE id = @id::uuid FOR UPDATE;
+
+-- name: AcquireStaleChatDiffStatuses :many
+WITH acquired AS (
+    UPDATE
+        chat_diff_statuses
+    SET
+        -- Claim for 5 minutes. The worker sets the real stale_at
+        -- after refresh. If the worker crashes, rows become eligible
+        -- again after this interval.
+        stale_at = NOW() + INTERVAL '5 minutes',
+        updated_at = NOW()
+    WHERE
+        chat_id IN (
+            SELECT
+                cds.chat_id
+            FROM
+                chat_diff_statuses cds
+            INNER JOIN
+                chats c ON c.id = cds.chat_id
+            WHERE
+                cds.stale_at <= NOW()
+                AND cds.git_remote_origin != ''
+                AND cds.git_branch != ''
+                AND c.archived = FALSE
+            ORDER BY
+                cds.stale_at ASC
+            FOR UPDATE OF cds
+                SKIP LOCKED
+            LIMIT
+                @limit_val::int
+        )
+    RETURNING *
+)
+SELECT
+    acquired.*,
+    c.owner_id
+FROM
+    acquired
+INNER JOIN
+    chats c ON c.id = acquired.chat_id;
+
+-- name: BackoffChatDiffStatus :exec
+UPDATE
+    chat_diff_statuses
+SET
+    stale_at = @stale_at::timestamptz,
+    updated_at = NOW()
+WHERE
+    chat_id = @chat_id::uuid;