Mattermost Packet O server-deep doc hotfix verified

BLUF: Verified opencode Packet O hotfix, then applied one remaining consistency correction in server-deep docs. No runtime code changed.

Confirmed fixed:

• Store SQL implementation path now points to server/channels/store/sqlstore/.
• ClientConfigWithComputed source path now points to server/channels/app/platform/config.go.
• Markdown fences balanced.

Additional Codex correction:

• README/file-upload/server-chain-matrix now distinguish primary API upload path App.UploadFile -> DoUploadFileExpectModification from alternate streaming UploadFileX path.
• Removed stale app/file.go:30 and app/file.go:72 references in server-chain-matrix.

Next recommended packet:

• Packet O2: Store SQL implementation + table/schema trace for post/user/session/file/thread domains.